# Luminance and illuminance relationship goals

### Luminance vs. Illuminance Relationship

Luminance and illuminance sound like similar terms in the lighting world but have vastly different meanings. Find out the difference here. deliver accurate illuminance and luminance measurements in a single reading. Scotopic vision refers . The inverse square law defines the relationship between the irradiance .. The goal of an optical bench is to provide repeatability . It is not. In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception.

They are just different animals. Even then, you have to specify the solid angle over which you are measuring the reflected light, No you don't, it cancels out. I specified lambertian so it's not. Sure but it still applies in this case. In fact, it's even distance independent even though it involves illuminance. I was talking about the incident light. Perhaps you are not familiar with a lambertian distribution or maybe we are not talking about the same thing.

When light falls on a lambertian surface, it reflects in all directions with intensity equal to the cosine of the angle intensity drops with the cosine of the reflected angle.

The larger the angle, the larger the area. That's where the two cosines cancel, resulting in equal luminance regardless of angle. I was talking about the incident light They are just different animals.

- Luminance vs. Illuminance
- Illuminance

When one measures the luminance of the reflected light, however, the surface area being mensured is dependent on that same angle of reflection, which increases with the cosine of the angle.

So when you tilt an illuminated lambertian surface, the brightness remains equal regardless of angle.

**Lighting terminology**

Try that with your favorite lambertian surface by tilting it, with a fixed source. When the source hits the part you observe at a small angle, the brightness goes down.

Perhaps you are thinking about the effect of equal brightness. Then the cosines cancel but this has nothing to do with the falling light, which is fixed for that effect. Going back to the OP, those are two different quantities, and you cannot "convert" one into the other.

## How to convert illuminance to luminance?

One has angular dependence, the other one does not. This is essential for photography since the solid angle is determined by the aperture. In the same way, you cannot convert seconds to meters but you can tell the distance traveled over an interval of time if you now the velocity. That would not be a "conversion". It would be an answer to a specific question.

### exposure - What is the difference between luminance and illuminance? - Photography Stack Exchange

Relation, but not a conversion is discussed here: This answers the question about the total luminance of a surface over all angles given the illuminance. The luminance itself not the total one is a different quantity not comparable to the illuminace.

My favorite is Barium Sulfate, and is actually the industry standard. One might look at that another way: Now that the definition of a steradian is out of the way, we can come to a clearer understanding of the relationship of a lumen to a candela.

According to the linked wikipedia article: A candela steradian is the luminous power of light emitted from a steradian, which as we now know from the discussion above, is the area of a circular patch on a sphere equal to the square of the radius of that sphere.

If we bring a light source into the discussion, to make things more real, that would translate into the following. Assuming we have a light bulb with a 1. The light meter will not measure If we assume our light meter is effectively sensitive to roughly one steradian, then it will measure 1 lumen. One may ask, why equate 1 lumen with 1 candela steradian, rather than just 1 candela? The answer is geometry.

Describing a candela is useful in telling us an amount of light, but not necessarily its concentration or the shape and size of the emission. The purpose of bringing steradians into the mix is that it involves a specific geometric shape and area to the light source emitting a lumen of light. It becomes more important when you have a high density light source.

### Illuminance - Wikipedia

Oh wait, you won't see! According to wikipedia, one lux is the measurement of lumens per square meter. In unit terms, since one lm is equal to one cd sr, then: The conversion of lux to EV is a fairly simple one that involves a constant C.