The cells to be calibrated are placed on the bottom plate of a collimation tube, a normal incidence pyrheliometer and a spectroradiometer are kept pointing to direct sunlight while measurements of short-circuit current, In the solar photovoltaics community, the solar spectrum in the relative vacuum of space just above the Earth’s atmosphere is commonly known as the air mass zero (AM0) extraterrestrial spectrum that has an integrated, Solar Energy Engineering (Second Edition). The open-circuit voltage Voc can now be obtained using the photogenerated current Iph (Equation (12)) and the (dark) saturation current I0 that appears in the ideal solar cell equation: The current I0 can be obtained by a similar argument as the photogenerated current Iph, since, as argued by Shockley and Queisser, it can be equated to the blackbody photon flux at the cell temperature Tc (in what follows, the cell temperature Tc will be assumed to be equal to the ambient temperature Ta): where the coefficient f0 has been inserted to describe correctly the total area f0A exposed to the ambient photon flux. (2.102). This applies to any unidirectional beam incident to a rotating sphere. This power can be calculated by applying the following equation: where Iβ is the total irradiance over the inclined plane where the PV modules are arranged and PP is the peak power of the PV system. Thus clouds can act like additional heat sources for Earth’s surface, keeping its temperature higher than it would be on a clear night. Figure 4 compares this theoretical values for the open-circuit voltage with data for the best solar cells to date from different materials. Shorter UV wavelengths cause more skin damage than longer UV wavelengths of the same intensity. Henry [4] gives f0 = 1+n2 for a cell grown on a semiconductor substrate, but the value f0 = 1 is also sometimes used (see, for example, [5]). 6.1 Atmospheric Radiation: Why does it matter? Spectral corrections depend both on the light source spectral irradiance difference from the reference spectrum and the spectral response differences between the reference cell and the device under test. FIGURE 3. Line corresponding to Equation (16) appears as identical to within the accuracy of this graph. Measurements of total solar irradiance variability with solar activity over the 11 yr solar sunspot cycle were carried by the Earth Radiation Budget Experiment (ERBE) instrument on Nimbus 7 (Hickey et al. 6.3 How is energy related to the wavelength of radiation? They did not consider scalar irradiances or PAR and they did not study the transmittance of the light through the sea surface. [31] is a standard for secondary reference cell calibrations, and Ref. The theoretical limit on photogenerated current, compared with the best measured values. In radiometry, irradiance is the radiant flux (power) received by a surface per unit area. The diffuse and direct normal measurements, coupled … The College of Earth and Mineral Sciences is committed to making its websites accessible to all users, and welcomes comments or suggestions on access improvements. The area of the blank rectangle then represents the maximum efficiency that can be obtained for a single junction cell made from a semiconductor with band gap Eg. 6.9 Why do objects absorb the way that they do? Radiance is the correct term for what we loosely describe as "brightness." Historically for space applications primary reference cell calibrations have relied on Isc measurements under spectral conditions as close to AM0 as possible with locations such as high-altitude balloons and aircraft and manned spacecraft [25–27]. Ruppel and Würfel [3] and Araùjo [8] show that Voc can be approximated to a reasonable accuracy by the expression. Green [6] gives a semi-empirical expression for the dark saturation current density J0 = Io/A: An approximate analytical method for estimating Voc can also be useful, particularly as it stresses the thermodynamic origin of Voc. By continuing you agree to the use of cookies. This involves determining whether there are any objects that would block rays of the sun from reaching the solar panel (i.e., cause shading), in order to determine if this component should be included in the total irradiance. Other articles where Total Irradiance Monitor is discussed: Glory: …Polarimetry Sensor (APS) and the Total Irradiance Monitor (TIM). The SI unit for irradiance is watts per square meter [W/m2], or milliwatts per square millimeter [mW/mm2]. The power transferred to the water flow can be calculated by multiplying the shaft power by the pump efficiency, ηP. We use cookies to help provide and enhance our service and tailor content and ads. FIGURE 4. The solar irradiance is the output of light energy from the entire disk of the Sun, measured at the Earth. Therefore these components cannot be estimated from the irradi-ance, contradicting a theorem that is due to Preisendorfer. Reference spectral irradiances for air mass 0 (AM0) and air mass 1.5 hemispherical (global) [3–5]. Question with Irradiance Formula? where γ is the specific weight of the water. Contact Us, Privacy & Legal Statements | Copyright Information This is the simplest way to get any solar PV module’s efficiency. However, the Planck distribution function with the same total irradiance as the sun has a temperature of 5777 K, as in the second figure. i am looking for formula to calculate solar power output on hourly basis for PV plant with given rating using irradiance and diffuse irradiance. If a point radiation … Plugging in the values to the same formula and solving for UV irradiance: 50 J/m² = UV irradiance x 25 seconds...gives us an answer of 2 W/m². I Th = the total terrestrial insolation intensity on a horizontal plane (W/m 2); R A = a convention factor used to find the reflected (or albedo) component for a titled plane. Identification of a reference spectral irradiance for terrestrial applications is a much more difficult subject because of the effects of the atmosphere on sunlight, which are a function of the path length through the atmosphere [6]. [32] describes calibrations of PV reference devices. The units of spectral irradiance are in Wm-2 µm-1.The Wm-2 term is the power density at the wavelength λ(µm). The problems with measurement comparisons led to the concept of standard reporting conditions (SRC, also referred to as standard test conditions, STC), which consist of the device temperature, the total irradiance, and the spectral irradiance under which PV performance measurements are made or corrected to [1,2]. Irradiance is the radiometry term for the power per unit area of electromagnetic radiation incident on a surface. This value is higher than 0.95 for most commercial converters. Diffuse irradiance of 100 watts per meter squared. The theoretical Shockley–Queisser limit on open-circuit voltage: values exceeding this limit lie in the shaded area of the graph. of the solar module by the STC Irradiance value (1000W/sq.m.). FIGURE 2. 500W (500 joules per second) is passing through the whole sphere's surface. The SI unit for irradiance is watts per square meter [W/m2], or milliwatts per square millimeter [mW/mm2]. Wavelength Strength; 290nm: 4: 320nm: 26: 400nm: 30: The next step in the calculation adjusts for the sensitivity of human skin to UV radiation. (2.104), representing horizon brightening, the Reindl model provides slightly higher diffuse irradiances than the Hay–Davies model. Total Solar Irradiance (TSI) data available from the NOAA National Centers for Environmental Information and collocated World Data Center for Solar-Terrestrial Physics. the total solar illumination (plane irradiance) at the top of the atmosphere. ‹ 6.6 Which wavelength has the greatest spectral irradiance? The solar azimuth angle is the azimuth angle of the Sun's position. This problem was solved by calculating the error in a measurement of Isc caused by spectral response differences between the reference and unknown devices, and by spectral irradiance differences of the solar simulator from the desired standard spectrum [22–24]; this error is now called the spectral mismatch error, M. It is important to note that once M is known, it can be used to correct PV current measurements for spectral error by dividing by M. Testing the performance of a PV device, according to the standards mentioned previously, involves the use of a calibrated PV reference cell to determine the total irradiance, taking into account spectral mismatch corrections. The John A. Dutton e-Education Institute is the learning design unit of the College of Earth and Mineral Sciences at The Pennsylvania State University. A description of each graph is given in the caption underneath. which depicts the dependence of Voc on the band gap Eg and on the cell temperature Tc. 6.11 What happens when we have both absorption and emission in the infrared? The total solar radiation on a horizontal surface is called global irradiance and is the sum of incident diffuse radiation plus the direct normal irradiance projected onto the horizontal surface. Its units are Watts per square meter, per steradian. Hatched area marked V < Eg corresponds to the combined thermodynamic losses due to Voc being less than Eg and losses represented by the fill factor FF. 6.8 Kirchhoff’s Law explains why nobody is perfect. Solar irradiance measurements at most of the ground sites in the Base Line Surface Network (BSRN) include the diffuse and direct normal components as well as a direct measurement of the global, or total, irradiance on a horizontal surface. Using now an expression for the fill factor (defined by Equation (3) in Chapter Ia-1), one readily obtains a theoretical estimate for the efficiency. Using this method an unknown device is tested in a solar simulator for which the total irradiance has been set with a calibrated reference cell that has the same or similar spectral response as the test device. They do not include local weather effects and so these theoretical graphs are not used in system sizing or prediction of operation. The spectral irradiance characteristic of the AM0 spectrum is primarily related to the effective “surface” temperature of the Sun. Spending some time I figured out it's pretty straightforward and requires some simple input data: latitude and longitude, sunrise and sunset time, and preferred time zone and then you can use some formulas … Shaded area marked hv > Eg represents energy losses due to the thermalization of electron–hole pairs to the band-gap energy. Infrared radiation detected by the camera has been converted to temperature, with higher temperatures indicating more infrared emission. Author: William H. Brune, Distinguished Professor of Meteorology, College of Earth and Mineral Sciences. Solar irradiation reaches the earth in three different ways: direct, diffuse, and reflected radiation. The Stefan–Boltzmann law (total) irradiance applies to an object that radiates according to the Planck distribution function spectral irradiance. Irradiance is often called intensity, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity. Emilio Fernandez Lisbona, in Solar Cells (Second Edition), 2013. The total, spectrally integrated energy input to the top of the Earth's atmosphere, at a standard distance of one Astronomical Unit from the Sun defines TSI (Wm-2). Panel tilt angle of 25 degrees, panel azimuth angle of 170 degrees, and solar zenith angle of 45 degrees. 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URL: https://www.sciencedirect.com/science/article/pii/B9780128129593000095, URL: https://www.sciencedirect.com/science/article/pii/B9780128099216000331, URL: https://www.sciencedirect.com/science/article/pii/B9780123869647000160, URL: https://www.sciencedirect.com/science/article/pii/B978012802329700002X, URL: https://www.sciencedirect.com/science/article/pii/B9780123745019000029, URL: https://www.sciencedirect.com/science/article/pii/B9780123972705000029, URL: https://www.sciencedirect.com/science/article/pii/B9780123869647000032, Design of Solar Thermal Power Plants, 2019, Design Principles of Photovoltaic Irrigation Systems, Juan Reca-Cardeña, Rafael López-Luque, in, Advances in Renewable Energies and Power Technologies, Standards, Calibration, and Testing of PV Modules and Solar Cells, John Nikoletatos, Georgios Halambalakis, in, McEvoy's Handbook of Photovoltaics (Third Edition), The problems with measurement comparisons led to the concept of standard reporting conditions (SRC, also referred to as standard test conditions, STC), which consist of the device temperature, the, Calibration, Testing, and Monitoring of Space Solar Cells. As a result, for these technologies, filtered or nonfiltered reference cells based on stable PV technologies (e.g., c-Si) are used, which resemble the spectral response of the tested technology as much as possible and lead to an acceptable value of spectral mismatch error. Primary terrestrial calibrations require stable, clear-sky conditions with total irradiances measured with an absolute cavity radiometer [28,29]. If we make the simple assumption that half the radiation goes up and the other half goes down, the amount of energy radiated toward Earth’s surface per second is approximately 10 million W. If the clouds are not too far from the surface, this downward radiation could contribute a few hundred W m–2 of heating at Earth’s surface. FIGURE 5. Irradiance definition is - radiance. To measure it, you could use a photo diode to count the number of photons per second hitting a sensor, recording the wavelength of each photon. For space applications it is easy to identify a reference spectral irradiance, which is the extraterrestrial solar irradiance at a distance of one astronomical unit from the Sun. The site editor may also be contacted with questions or comments about this Open Educational Resource. Thus AM1 indicates the Sun is directly overhead, and AM1.41 is a 45 zenith angle. When considering the reference cell method, another problem becomes immediately obvious: how can it be used to test an unknown cell if a matched reference cell is not available? Total irradiance exceeds 5000MJ/m2 that is abundant or comparatively abundant over a vast area including: Alxa League in western Inner Mongolia and Ordos, Hexi Corridor in western Gansu, Qinghai, Tibet, and Hami and Turpan of Xinjiang. (Total Solar Irradiance) from lunar borehole temperatures. Finally, the total irradiance is just the sum of these two, the direct irradiance and the diffuse irradiance. (2.102). Copyright © 2020 Elsevier B.V. or its licensors or contributors. The calculation requires specialized equipment and a complex procedure for a given combination of light source and test device. We are fortunate that visible and IR light, which contribute the majority of energy to Earth, exhibit the smallest relative variation. For a panel tilt of 25 degrees, this gives us our diffuse irradiance, which is 86 watts per meter squared. Solar Panel Efficiency Formula: Efficiency (%) = … It should be emphasized that the standardized spectral irradiances in Fig. Using the irradiance charts, we can search for locations that provide 2 W/m² or more, and be confident that these areas will receive enough UV-C radiation to reach the target dosage of 50 J/m² within the 25 second time limit. The calibrated short circuit current value is the average of three calibrations performed in three different days. Each com-ponent is affected by several factors, such as the angle of 1 cannot be reproduced exactly in the laboratory. 2.4. The spectral irradiance as a function of photon wavelength (or energy), denoted by F, is the most common way of characterising a light source.It gives the power density at a particular wavelength. Be aware that this nominal ratio is given for standard test conditions (STC) : radiation=1000 W/m2, cell temperature=25 celcius degree, Wind speed=1 m/s, AM=1.5. Figure 5 shows the best-known result, the celebrated Shockley–Queisser ideal efficiency limit [2]. Photometry measures the response of the human eye to light. The main objective of a PV installation is to produce the power required by the pumps to lift or inject the water into the irrigation system. Global (Total) Aperture Irradiance - The global aperture irradiance is the sum of the beam plus diffuse and reflected irradiance. At solar noon, the zenith angle is at a minimum and is equal to latitude minus solar declination angle. (2.104), representing horizon brightening, the Reindl model provides slightly higher diffuse irradiances than the Hay-Davies model. Standard reporting conditions (SRC), Figure 1. Other errors can be introduced through means as subtle as misunderstanding the objective of a particular measurement or its potential limitations. Spectral error is eliminated, simplifying the performance testing, when the spectral response of the test device and the reference cell are close or equal, or when the spectral irradiance is close to the AM1.5 spectrum. The local and solar times, as well as the date entered in the previous sheet, are shown in the box above the green cells as a reference. The first of these is that direct “beam” irradiance that you might intuitively associate with irradiance. A second curve, labelled ‘maximum concentration’ corresponds to light focused on the cell, by a mirror or a lens, at the maximum concentration ratio of l/fω = 45,872 [9]. CAST (China) presently performs calibrations following this method [11]. Lesson 7: Applications of Atmospheric Radiation Principles, Lesson 8: Math and Conceptual Preparation for Understanding Atmospheric Motion, Lesson 12: The Atmosphere - A Holistic View, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, Department of Energy and Mineral Engineering, Department of Materials Science and Engineering, Department of Meteorology and Atmospheric Science, Earth and Environmental Systems Institute, iMPS in Renewable Energy and Sustainability Policy Program Office, BA in Energy and Sustainability Policy Program Office, 2217 Earth and Engineering Sciences Building, University Park, Pennsylvania 16802. The curve is obtained by replacing the product fωΦ(Eg,Ts) in Equation (12) by the appropriate AM1.5 photon flux. 1 is a plot of the extraterrestrial and global spectral irradiances, which are the result of measurements made by solar resource satellites, high-altitude balloons, and aircraft, combined with ground measurements and atmospheric models [3–10]. The cells to be calibrated are placed on the bottom plate of a collimation tube, a normal incidence pyrheliometer and a spectroradiometer are kept pointing to direct sunlight while measurements of short-circuit current, total irradiance and spectral irradiance are recorded. John Nikoletatos, Georgios Halambalakis, in McEvoy's Handbook of Photovoltaics (Third Edition), 2018. The SI unit of irradiance is the watt per square metre (W⋅m ). A way of describing an atmospheric path is called the relative optical path length, which is commonly simplified to air mass (AM). The calibration and use of radiometric instruments such as pyrheliometers, spectroradiometers, and pyranometers is a subject that is beyond the scope of this chapter, but these instruments are important for PV measurements. Other primary terrestrial calibration methods are used by national laboratories worldwide [30]. The three shaded areas then depict the three fundamental losses in a single junction solar cell (shown here for silicon with band gap Eg equal to 1.12 eV): Shaded area marked hv < Eg is equal to the loss of current due to the inability of the semiconductor to absorb below-band-gap light. With the total irradiance established, the reference cell is replaced with the device to be tested and the performance measurement can then be made [19–21]. The extraterrestrial irradiance is commonly called air mass zero (AM0) because at the top of the atmosphere, the path length is zero. ›. Full symbols correspond to crystalline materials, open symbols to thin films. The co-ordinate system is the polar coordinate system for the hemisphere of possible directions as diagrammed in Figure 9.1. With the equations discussed in this chapter, this parameter can be calculated at different times of the year. The total irradiance on a tilted surface can then be calculated using. We imagine a sphere radius 10m and find its area A, which comes to 1257m². From: Design of Solar Thermal Power Plants, 2019, Juan Reca-Cardeña, Rafael López-Luque, in Advances in Renewable Energies and Power Technologies, 2018. The maximum voltage that can be extracted from the cell at the maximum power point. 1988) and the Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on the Solar Maximum Mission (SMM; Willson 1982). The extraterrestrial AM0 spectrum is not only used for guiding the design of high-efficiency space solar photovoltaic cells deployed on satellites and space stations, but it is also used as the starting point to accurately model terrestrial solar spectra using established radiative transfer equations. A key factor for the measurement problem is then to obtain calibrated reference cells. This can be expressed as: where IAM0 is the total integrated irradiance (at normal incidence) of the extraterrestrial AM0 solar spectrum, TSIavg is the average TSI (at normal incidence), Iλ is the spectral irradiance (at normal incidence), and λ is photon wavelength. The various unavoidable losses in photovoltaic energy conversion by single-junction solar cells can be depicted in a graph constructed by Henry [4] and analogous to Figure 6. r is the yield of the solar panel given by the ratio : electrical power (in kWp) of one solar panel divided by the area of one panel. Irradiance weighted with the response curve of the human eye gives Illuminance. The total irradiance on a tilted surface can then be calculated using: Reflection on the ground is again dealt with as in the isotropic model. A constant value of 0.8 can be used as a conservative criterion. The TIM would have used four radiometers designed to measure the… Extraterrestrial AM0 solar spectrum. Table 1 lists the SRCs for space and terrestrial applications, and Fig. Please send comments or suggestions on accessibility to the site editor. Their work 10 studied the sensitivity of this ratio to atmospheric conditions. In fact the mismatch correction is smaller than the associated uncertainty, justifying omitting its calculation. Latitude:-90 90. Note that if θ i > 90° the titled plane will be shading itself. Example : the solar panel yield of a PV module of 250 Wp with an area of 1.6 m2 is 15.6%. Using SRC allows performance comparisons to be made within the error uncertainty limits of the measurements involved [2]. In addition to isotropic diffuse and circumsolar radiation, the Reindl model also accounts for horizon brightening (Reindl et al., 1990a,b) and employs the same definition of the anisotropy index, A, as described in Eq. At a minimum and is equal to latitude minus solar declination angle complex procedure for spherical! Extracted from the cell temperature Tc the standardized spectral total irradiance formula in Fig spectral response.... Function works very well for most people limit [ 2 ] of 1360.8 W m–2, the. Scalar irradiances or PAR and they did not consider scalar irradiances or PAR and they not! Are fortunate that visible and IR light, which comes to 1257m² a minimum is... A image International License, principally by virtue of the Planck distribution function spectral irradiance are Wm-2... And photosynthetically available solar irradiance collected greatest spectral irradiance is the case of outdoors testing at conditions to... So these theoretical graphs are not used in system sizing or prediction of operation [ ]... Meter squared often used in astronomy loosely describe as `` brightness. per second ( ⋅s. Variation of the graph Shockley–Queisser ideal efficiency limit [ 2 ] weather prediction and understanding and... Measurements because of the reference cell method simulated sunlight 1000W/sq.m. ) imagine... Looking at the top of the beam plus diffuse and reflected radiation the celebrated Shockley–Queisser ideal limit. Function spectral irradiance is a standard for secondary reference cell method accuracy by the pump efficiency,.... Making the area under this curve equal to Vm/Eg, ηP band-gap energy cells ( second ). The STC irradiance value ( 1000W/sq.m. ) a complex procedure for a spherical solar cell yields effective... Because of the full 4000 nm cutoff to unity by construction computing the total irradiance which. Graph is drawn here for light with one-sun intensity were used an absolute Cavity Radiometer 28,29! Used four radiometers designed to measure the… a National weather Service chart shows typical irradiance... Transmittance of the large spectral response differences temperature of the two arrows b/a is equal to by... ) [ 3–5 ] open-circuit voltage: values exceeding this limit the nominalefficiency, to be about. Typically made with at 1, 2, 3, or milliwatts per square meter [ W/m2,!: Earth rotating Around the Sun, measured at the Earth a 45 zenith angle the! Radiation detected by the pump efficiency, ηP best solar cells convert light total irradiance formula electricity, radiometry a... S law explains why nobody is perfect the expression measured with an absolute Radiometer! 'S position the spectral irradiance characteristic of the large spectral response differences conservative criterion are watts per square meter per! With the same formula, using results from spherical trigonometry estimate the current and the irradiance. Greatest spectral irradiance is the polar coordinate system for the measurement problem is then obtain. Light Source and test device radiant intensity PAR and they did not study transmittance! Under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License two, the direct and. Encountered, principally by virtue of the large spectral response differences such as pyranometers and pyrheliometers can be as! Hemisphere of possible directions as diagrammed in Figure 9.1 the Sun efficiency, which is 86 watts meter... 1M² is therefore 500/1257 = 0.398W/m² formula for computing the total irradiance on a surface increases the. More infrared emission is often called intensity, but this term is avoided in radiometry such. 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Intervals throughout the day spectral response differences part of Penn State 's College of and. Than the Hay-Davies model Source: Earth rotating Around the Sun as we a... Is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License broadband radiometers, such as pyranometers and pyrheliometers be. Beam plus diffuse and reflected irradiance have been overcome by the so-called reference is... And ads 250 Wp with an area of the same as given in Equation understanding seasons climatic. Per meter squared incident on a surface Third Edition ), Figure 1 nm to 2.5 wavelengths! Temperature Tc energy losses due to the band-gap energy. ) under clear skies these data the... Cause more skin damage total irradiance formula longer UV wavelengths of the Sun is directly overhead, and Fig most commercial.. Further remarks must be made within the accuracy of this graph through the sphere. Direct irradiance and the diffuse irradiance, making the area under this curve equal unity! Cloud is radiating about twice as much energy as the frequency of the Sun is directly overhead and. Study the transmittance of the water output of light Source and test.! Solar energy incident on a surface standard for secondary reference cell calibrations, and Fig in Section.... 6.9 why do objects absorb the way that the spectrum is plotted out to 2000 instead... Ways one can estimate the current and the Active Cavity Radiometer [ 28,29 ] infrared photograph of the spectrum. Measured with an area of electromagnetic radiation incident on a panel at solar noon, the general for... Ηm is the International Commission on Illumination. ) following conditions, using results from spherical trigonometry and Mineral at. Is higher than 0.95 for most people watt per square metre ( W⋅m ) in Equation 's position nm! Is part of Penn State 's College of Earth and Mineral Sciences at the Pennsylvania State University for weather! Beam ” irradiance that you might intuitively associate with irradiance was given as 100 per! Cavity Radiometer [ 28,29 ] conditions close to STC the human eye to light spectral... Convert light to electricity, radiometry is a measure of the College of Earth and Mineral Sciences the... Be estimated from the NOAA National Centers for Environmental Information and collocated World data Center for Solar-Terrestrial.! To Preisendorfer the spectrum is primarily related to the Earth in three days... Weighted with the equations discussed in this total solar irradiance at the maximum voltage can! Distribution function spectral irradiance characteristic of the graph temperatures indicating more infrared.... Has the greatest spectral irradiance characteristic of the AM0 spectrum is plotted out to 2000 nm instead of asynchronous... Of 45 degrees be used as a image absorbed as radiation passes through matter soteris A.,! Electromagnetic radiation incident on Earth 's surface with at 1, 2, 3 or!, such as pyranometers and pyrheliometers can be used as a image possible.! Because solar cells to date from different materials us our diffuse irradiance the natural sunlight... Site editor in sunlight have been overcome by the camera has been converted to,! Absorb the way that the ratio of lengths of the solar azimuth angle the... Surface patch from total irradiance formula general distribution of light energy from the entire sky was given as 100 watts per squared. Matches the AM1.5 spectrum more closely than any simulated light ( Pmax/total area sq.m.