# Team:Cambridge/Tools/Lighting

(Difference between revisions)
 Revision as of 15:46, 27 October 2010 (view source)Willh (Talk | contribs) (→Radiation out)← Older edit Latest revision as of 16:35, 27 October 2010 (view source)Willh (Talk | contribs) (5 intermediate revisions not shown) Line 85: Line 85: *'''Luminance''', which is measured in candelas per square metre (cd m-2). *'''Luminance''', which is measured in candelas per square metre (cd m-2). - If you look at a black object and ask "how much light is coming out of that?" there are two answers. The first is obviously none, since it appears black, but the second relies on the fact that it is possible that the object is very hot and is emitting in the infra-red, or is a source of UV and is thus emitting a lot of light. The two measures above quantify this difference. Radiance measures the actual electromagnetic radiation coming from an object, whereas luminance adjusts for the perceptive abilities of the human eye. If you look at figure 3 you can see the '''luminosity functions''' which supply this weighting in the visible part of the spectrum. + {{:Team:Cambridge/Templates/RightImage|image=photopicscotopic.png|caption=''Figure 2: Photopic (black) and scotopic (green) luminosity functions.''}} - {{:Team:Cambridge/Templates/RightImage|image=photopicscotopic.png|caption=''Figure 3: Photopic (black) and scotopic (green) luminosity functions.''}} + If you look at a black object and ask "how much light is coming out of that?" there are two answers. The first is obviously none, since it appears black, but the second relies on the fact that it is possible that the object is very hot and is emitting in the infra-red, or is a source of UV and is thus emitting a lot of light. The two measures above quantify this difference. Radiance measures the actual electromagnetic radiation coming from an object, whereas luminance adjusts for the perceptive abilities of the human eye. If you look at figure 2 you can see the '''luminosity functions''' which supply this weighting in the visible part of the spectrum. There are two curves, one for photopic vision, and the latter for scotopic. The former is normal vision in good light and covers the normal range of colours. The latter is for dark vision, when the (colour percieving) cone cells are unable to function. This part of the reason why you can only see in black and white in the dark. There are two curves, one for photopic vision, and the latter for scotopic. The former is normal vision in good light and covers the normal range of colours. The latter is for dark vision, when the (colour percieving) cone cells are unable to function. This part of the reason why you can only see in black and white in the dark. + + {{:Team:Cambridge/Templates/RightImage|image=Luminosityfunction.png|caption=''Figure 3: The formula for calculating luminance. (F:Luminous Flux, y:Luminosity Function,J:Spectral Power Distribution,λ:Wavelength''}} To convert from Radiance to Luminance you integrate the power spectrum weighted by the luminosity function so that wavelengths beyond that of human perception are cut out. Note that this transformation is therefore one-way. You can't convert from Luminance to Radiance. To convert from Radiance to Luminance you integrate the power spectrum weighted by the luminosity function so that wavelengths beyond that of human perception are cut out. Note that this transformation is therefore one-way. You can't convert from Luminance to Radiance. Line 95: Line 97: The measurements of light output taken above are in '''lumens''' (a measurement of Luminance) where 1lm=1cd*sr. The lumen thus quantifies the human-percieved total amount of light being emitted from an object. The above values use the scotopic luminosity function, since street lights operate in low-light conditions. The measurements of light output taken above are in '''lumens''' (a measurement of Luminance) where 1lm=1cd*sr. The lumen thus quantifies the human-percieved total amount of light being emitted from an object. The above values use the scotopic luminosity function, since street lights operate in low-light conditions. - {{:Team:Cambridge/Templates/RightImage|image=Luminosityfunction.png|caption=''Figure 4: The formula for calculating luminance. (F:Luminous Flux, y:Luminosity Function,J:Spectral Power Distribution,λ:Wavelength''}} + {{:Team:Cambridge/Templates/RightImage|image=Fischerispectrum.jpg|caption=''Figure 4: The emission spectrum of V. Fischeri (shown in black)''}} - We wrote a program ([[Team:Cambridge/luminanceSourceCode | source code]]) in c++ which allowed the user to input their own power spectrum and be told the resulting luminance measure. By inputting the curve shown in figure 2 which details the emission spectrum of the Vibrio Fischeri we found the formula: + We wrote a program ([[Team:Cambridge/luminanceSourceCode | source code]]) in c++ which allowed the user to input their own power spectrum and be told the resulting luminance measure. By inputting the curve shown in figure 4 which details the emission spectrum of the Vibrio Fischeri we found the formula: Total Bacterial Luminance(lm)=471.13 x Total Energy Output(W) Total Bacterial Luminance(lm)=471.13 x Total Energy Output(W) - It should be noted that this is actually a really good conversion factor, only about 33% of the radiant energy is lost to the outer regions of human perception (at best 1lm = 683.002 W). This is due to the fact that (as you can see from the scotopic luminosity function in figure 3) the human eye is better at seeing blue light in the dark than red light, and our bacterial light is clearly blue. + It should be noted that this is actually a really good conversion factor, only about 33% of the radiant energy is lost to the outer regions of human perception (at best 1lm = 683.002 W). This is due to the fact that (as you can see from the scotopic luminosity function in figure 2) the human eye is better at seeing blue light in the dark than red light, and our bacterial light is clearly blue. - + - {{:Team:Cambridge/Templates/RightImage|image=Fischerispectrum.jpg|caption=''Figure 2: The emission spectrum of V. Fischeri (shown in black)''}} + ==Bringing it all together== ==Bringing it all together== Line 111: Line 111: '''Efficiency = (X*Tnight) / (471.13*60A*Tday)''' '''Efficiency = (X*Tnight) / (471.13*60A*Tday)''' - where '''Tday''' and '''Tnight''' are the hours of daylight and night time respectively. If we choose the least bright street lamp ('''X=210''') and hypothesise a projected area of '''A=30m2''', and a '''day:night ratio 14:10'''  then we find that the efficiency must be roughly '''0.02%'''. This means that 0.02% of the total energy hitting the tree must be converted eventually into light output, a potentially achievable target. + where '''Tday''' and '''Tnight''' are the hours of daylight and night time respectively. If we choose the least bright street lamp ('''X=210''') and hypothesise a projected area of '''A=30m2''', and a '''day:night ratio 14:10'''  then we find that the efficiency must be roughly '''0.02%'''. This means that 0.02% of the total energy which the tree absorbs in photosynthesis must be converted eventually into light output, a potentially achievable target. {{:Team:Cambridge/Templates/footer}} {{:Team:Cambridge/Templates/footer}}