# Help Understanding Luminance Calculation Process

• I
• KLoux
In summary, luminance is the measure of the amount of light emitted or reflected from a surface and is important in calculations because it helps determine the perceived brightness of an object or image. It is calculated by multiplying the luminance factor by the illuminance, and can be measured in units such as candelas per square meter, foot-lamberts, and nits. Factors such as surface reflectance, light source angle and intensity, and human perception can affect the accuracy of luminance calculations. Luminance calculations are commonly used in lighting design, photography, and display technology.
KLoux
Hello all,

I have a system that consists of:
• A projector with a known position, orientation, and brightness (luminous flux, I guess - units are lumens),
• A lens with known distortion characteristics (i.e. a known light ray vector for every point on the projector's image-generating panel)
• A projection surface with known geometry (not necessarily a planar surface) and assumed reflectivity, R
• An eye point
I am trying to calculate the luminance at the eye point for a small light ray emanating from the projector. I am defining the "small light ray" by choosing four corners of a square on the projector's panel. The geometry is no problem - I am calculating the corresponding vectors for the light leaving the lens and their intersection with the projection surface (call these points ##p_1##through ##p_4##). My question is related to the process of using this information to arrive at a luminance value.

I am using the following process:
1. Calculate luminous flux for the small light ray (##\phi_{V0}##) by dividing the area of the square on the panel by the total panel area and multiplying by projector luminous flux (##\phi_{V0} = \frac{A_0}{A} \phi_V##) in lumens.
2. Average the angles of incidence where each of the four light direction vectors intersect the projection surface and evaluate the cosine of this angle (##\cos(\theta_0)##).
3. Calculate the luminous flux of the light reflecting off of the projection surface (perpendicular to the surface) by multiplying the quantities above (##\phi_{V1} = \phi_{V0} \cos(\theta_0)## in lumens).
4. Average the viewing angles of incidence (vectors from the eye point to ##p_1## through ##p_4##) and evaluate the cosine of this angle (##\cos(\theta_1)##).
5. Calculate the solid angle subtended from the eye point toward the area bounded by ##p_1## through ##p_4## (##\Omega##) in steradians.
6. Average the vectors from the eye point to ##p_1## through ##p_4##; this average direction vector defines the normal to a plane. Project the area of the projection surface bounded by ##p_1## through ##p_4## onto this plane and compute the area (##A_P##) in m2.
7. Compute the luminance as ##L_V = \frac{\phi_{V1} R \cos(\theta_1)}{\Omega A_P} ## in cd/m2.
I've tried to illustrate these steps in the following images.
Steps 2 and 3:

Step 4:

Step 5:

Step 6:

I am assuming that the "small light ray" is small enough that the edges of the points of intersection with the projection surface can be connected by straight lines without great loss of accuracy. I intended to also assume perfect diffusion of light by the projection surface. I believe this is done by including the two cosine terms, but I haven't found examples that go from light source to surface to eye point, so I'm not 100% sure if including two cosine terms is correct. Several sources have stated that for a perfect diffuser, luminance is the same in all directions (implying that maybe I shouldn't need the second cosine term), but the accompanying images seem to show that the direction is important (i.e. different magnitude vectors in different directions), like this image (from https://www.bksv.com/media/doc/18-231.pdf):

I know from measurements on the physical system that I should expect the luminance to be ~10 cd/m^2, but my calculations yield much larger numbers (off by ~3 OOM). I believe the arithmetic is correct, so I assume that the error is in my understanding of luminance. Maybe I'm using the wrong solid angle or projected area?

Thanks,

Kerry

Last edited:

Dear Kerry,

Thank you for sharing your detailed process for calculating luminance at a specific point in a projection system. It seems like you have a good understanding of the various components involved, such as the projector, lens, projection surface, and eye point. Your assumptions of perfect diffusion and straight-line connections for the small light ray are also valid.

From your description, it seems like you are on the right track with your calculations. However, I would like to offer a few suggestions that may help you to refine your process and potentially address the discrepancy in your results.

Firstly, when calculating the luminous flux for the small light ray, you mentioned using the area of the square on the panel and the total panel area. It may be beneficial to also consider the brightness (luminous flux) of the individual pixels on the panel, as this could affect the overall luminous flux of the small light ray.

Secondly, in step 4, you mentioned averaging the viewing angles of incidence and evaluating the cosine of this angle. It may be more accurate to calculate the luminous flux for each individual viewing angle and then average the results. This could account for any variations in luminous flux at different viewing angles.

Lastly, in step 6, when calculating the solid angle subtended from the eye point, make sure to take into account the size and shape of the projection surface. If the surface is not a perfect plane, this could affect the calculation of the solid angle and the resulting luminance value.

I hope these suggestions are helpful to you and that you are able to refine your process and arrive at a more accurate luminance value. Keep up the good work!

## 1. What is luminance and why is it important in scientific calculations?

Luminance is the measure of the amount of light that is emitted or reflected from a surface. It is important in scientific calculations because it helps us understand how light interacts with different materials and how it affects our perception of brightness and color.

## 2. How is luminance calculated?

Luminance is calculated by measuring the amount of light energy per unit area, also known as luminous flux, and dividing it by the area of the surface. This results in a unit of measure called candela per square meter (cd/m²).

## 3. What factors affect luminance calculations?

The factors that affect luminance calculations include the amount and type of light source, the reflectivity of the surface, and the distance between the light source and the surface. Other factors such as atmospheric conditions and the angle of observation can also have an impact on luminance calculations.

## 4. How is luminance used in practical applications?

Luminance is used in a variety of practical applications, such as lighting design, display technology, and color measurement. It is also important in industries such as photography, cinematography, and vision science, where understanding the perception of light is crucial.

## 5. What are some common units of measure for luminance?

Some common units of measure for luminance include candela per square meter (cd/m²), foot-lamberts (fL), and nit (nt). These units may be used in different industries or regions, but they all measure the same concept of luminance.

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