NCEES Problem 123: Illumination

[jd5191]

I'm quite weak at Illumination in general, but the formulas in the handbook seem pretty straightforward. For the below problem, I would have assumed we use the formula for flux from the handbook:



which requires Initial Illumination level as an input. I thought the MMI=50fc we would maybe ignore depreciation factors for this equation since they're not provided. However in the solution the 'required illumination level' is set to the 'initial illumination level' in the formula. Could someone help me understand?

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[akyip]

I'm quite weak at Illumination in general, but the formulas in the handbook seem pretty straightforward. For the below problem, I would have assumed we use the formula for flux from the handbook:

View attachment 20302

which requires Initial Illumination level as an input. I thought the MMI=50fc we would maybe ignore depreciation factors for this equation since they're not provided. However in the solution the 'required illumination level' is set to the 'initial illumination level' in the formula. Could someone help me understand?

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I don't have an explanation for the NCEES formulas, but typically for such lighting problems I just use:

Lumens = (E * A) / (CU * LLF * BF)

E is just your given illumination in fc or lux

A is room area, in ft^2 or m^2

CU is coefficient of utilization.

LLF is light loss factor. If not specified, you can just assume this to be equal to 1.

BF is ballast factor. Again if not specified, you can just assume this to be equal to 1.

Also, another formula I've seen for luminous flux is:

Lumens = (E * A) / (CU * MF)

MF = LLF * BF = maintenance factor. Same deal as LLF and BF: if not specified, assume MF = 1

 

[DilutedAr18_PE]

I'm quite weak at Illumination in general, but the formulas in the handbook seem pretty straightforward. For the below problem, I would have assumed we use the formula for flux from the handbook:

View attachment 20302

which requires Initial Illumination level as an input. I thought the MMI=50fc we would maybe ignore depreciation factors for this equation since they're not provided. However in the solution the 'required illumination level' is set to the 'initial illumination level' in the formula. Could someone help me understand?

View attachment 20300

View attachment 20301

The formula you actually want to use is the first formula of page 25 of the handbook, which has the “initial flux in lumens” as a variable.  Since they do not give you a LLF, that is 1.0. “# of Luminaires” and “lamps/luminaire” can be assumed as 1.0, since the problem does not give you any of those variables.

You could manipulate those formulas you listed to get the correct answer, but the other formula already has it all together for you already. 

 

[Dothracki PE]

I too fell victim to this confusion with the reference book in my practice exams. 

I think those two problems really need to be combined in some way. As @akyipmentioned, the lumen equation should include both the coefficient of utilization and loss factors, but it is not included in the handbook.

This might be one of the formulas on there that I would suggest memorizing the modification to match the formula @akyip listed. 

Just make sure to be really careful with the terms given in the problem and the reference book and cross reference each letter representation with the legend and make sure you have the right formula given the problem information and what the problem is asking for. 

The first equation might only be used if the problem is specifically asking about initial or maintained footcandle level. 

 

[jd5191]

Lumens = (E * A) / (CU * LLF * BF)

@akyip Thanks, this is the formula I saw in my studies, but I'm still trying to make sense of the equations in the handbook because I will reference this section if I get an illumination level question.

The formula you actually want to use is the first formula of page 25 of the handbook, which has the “initial flux in lumens” as a variable.  

@DilutedAr18_PEwhich formula exactly? The first formula on pg 25 is LLF = LLD * LDD

The first equation might only be used if the problem is specifically asking about initial or maintained footcandle level. 

@DothrackiI'm glad someone else was a little confused by this, I've always been told if you know the illumination formulas, its pretty easy and without a background in illumination, I was hoping that's all I would have to do. Do you know if you are asked a questions about initial illumination, do you actually use the Coefficient of Utilization twice if you want to get flux? I.e. first in the Initial Illumination formula and then again in the luminous flux formula?

 

[Dothracki PE]

I'm glad someone else was a little confused by this, I've always been told if you know the illumination formulas, its pretty easy and without a background in illumination, I was hoping that's all I would have to do. Do you know if you are asked a questions about initial illumination, do you actually use the Coefficient of Utilization twice if you want to get flux? I.e. first in the Initial Illumination formula and then again in the luminous flux formula?

I'm really just speculating, so maybe don't listen to me. But it could be possible that you would need to use the CU twice if you were given MMI and asked to find the initial flux value.

As for the example problem you show, it states "required" illumination level is given. So I would just use that second formula of the reference sheet, which really should be like the first formula below. The second formula is take from another source for comparison. Since we aren't given a number fixtures or lamps per fixture, those are excluded. But if you rearrange to solve for lumens, you get the same equation as the first formula.

 

[DilutedAr18_PE]

@akyip Thanks, this is the formula I saw in my studies, but I'm still trying to make sense of the equations in the handbook because I will reference this section if I get an illumination level question.

@DilutedAr18_PEwhich formula exactly? The first formula on pg 25 is LLF = LLD * LDD

@DothrackiI'm glad someone else was a little confused by this, I've always been told if you know the illumination formulas, its pretty easy and without a background in illumination, I was hoping that's all I would have to do. Do you know if you are asked a questions about initial illumination, do you actually use the Coefficient of Utilization twice if you want to get flux? I.e. first in the Initial Illumination formula and then again in the luminous flux formula?

Sorry, that was supposed to be page 26. It’s essentially the same formula @Dothrackimentioned in the last post.