BebeshKing PE
THIS IS THE WAY
What would you answer for this kind of qualitative problem and why?
Illumination Qualitative problems
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Part of my job is to lay out and specify lights for buildings, so I have experience with photometrics and illumination. I hope the explanation below helps.
First I would eliminate B and C, since the width of luminaires never has any bearing on light level/illumination calculations. And B just doesn't make sense. You usually have 3 cavities to consider: The floor cavity, room cavity and ceiling cavity.
The zonal cavity method introduces two modification factors: coefficient of utilization (CU) and light loss factor (LLF)
CU is calculated based on total lamp lumens based on room size, configuration, surface reflectance and performance of fixtures (lumen output).
LLF accounts for light losses during depreciation. For example, when T8 fluorescent lamps were popular, you could assume after a few years the output of the lamps would drop as low as 85% of their original output, so you could add this multiplier when calculating total footcandles in a space. For LED's, it's closer to 95% since LEDs don't deprecriate as much as fluorescent bulbs.
Since surface reflectance is part of the CU method, the correct answer could be A or D. The reflectance of a surface will ultimately determine the footcandles you get on a surface. For example, a white acoustical tile ceiling will reflect much more light down to the floor than a ceiling painted black.
Usually we figure a standard 80-50-20 for reflectances (80% ceiling, 50% walls and 20% floors), but these can change if tile floors are replaced with dark carpet, the ceiling is painted black, walls are painted, etc.
In most cases, footcandle levels are calculated for either the floor surface or workplane height (3.5 feet). However, you're never concerned about workplane reflectance, since there's not much to influence light levels (a few desks and tables compared to an entire surface like a floor or ceiling). Therefore, I would choose answer A.
First I would eliminate B and C, since the width of luminaires never has any bearing on light level/illumination calculations. And B just doesn't make sense. You usually have 3 cavities to consider: The floor cavity, room cavity and ceiling cavity.
The zonal cavity method introduces two modification factors: coefficient of utilization (CU) and light loss factor (LLF)
CU is calculated based on total lamp lumens based on room size, configuration, surface reflectance and performance of fixtures (lumen output).
LLF accounts for light losses during depreciation. For example, when T8 fluorescent lamps were popular, you could assume after a few years the output of the lamps would drop as low as 85% of their original output, so you could add this multiplier when calculating total footcandles in a space. For LED's, it's closer to 95% since LEDs don't deprecriate as much as fluorescent bulbs.
Since surface reflectance is part of the CU method, the correct answer could be A or D. The reflectance of a surface will ultimately determine the footcandles you get on a surface. For example, a white acoustical tile ceiling will reflect much more light down to the floor than a ceiling painted black.
Usually we figure a standard 80-50-20 for reflectances (80% ceiling, 50% walls and 20% floors), but these can change if tile floors are replaced with dark carpet, the ceiling is painted black, walls are painted, etc.
In most cases, footcandle levels are calculated for either the floor surface or workplane height (3.5 feet). However, you're never concerned about workplane reflectance, since there's not much to influence light levels (a few desks and tables compared to an entire surface like a floor or ceiling). Therefore, I would choose answer A.
BebeshKing PE
THIS IS THE WAY
@MEtoEE, thank you for the explanation.
If the “coefficient of utilization“ will be on the choices, would you think it will the answer too?.
Usually if CU appears on an exam, the actual value is given to you. Most of the "illumination" problems are a matter of knowing what to plug in the formulas.
For qualitative problems like the one you mentioned, it's best to know what it is and how it's used, hence my explanation above. There are basically two ways to calculate lumens: The "Lumen (also called zone cavity) method", which is most common for interior lighting, and the "point to point" method, most common for exterior lighting.
For qualitative problems like the one you mentioned, it's best to know what it is and how it's used, hence my explanation above. There are basically two ways to calculate lumens: The "Lumen (also called zone cavity) method", which is most common for interior lighting, and the "point to point" method, most common for exterior lighting.
