How exactly does an incident light meter determine the exposure?

Question

I am struggling to understand what is going on inside the logic of an incident light meter in order to determine the exposure as opposed to a reflective light meter.

I understand that a reflective light meter assumes the measured spot to represent a middle gray tone, and accordingly displays exposure settings that will render the measured portion of the frame as a middle gray tone.

Now, when using an incident light meter, the meter should assume that the measured light is the same as that reflected by the subject. In order to provide a reading that will also render an 18% reflective object a middle gray tone in the photography, is it going to calculate that the measured incident light will be reflected by 18% by the subject, thus mulitplying the measured light intensity with 0.18 before treating it like a reflective light measurement?

In other words, assuming equal lighting conditions and an 18% gray card as a subject, in order for an incident and a reflective light metering to yield equal exposure settings, am I correct in assuming that the only difference of both modes is that in incident mode, a light meter needs to account for an assumed reflection of 18%, but otherwise works the same way as in reflective mode?

In this case, a light meter that is suitable for both modes would need to know what kind of measurement it is doing in order to yield correct results. For instance, according to the manual of Gossen Sixtomat F2, the only thing that a user needs to do to change between modes is to move the white dome accordingly. I assume that internally, there are switches to determine the position of the dome for the meter's electronics to distinguish between modes.

I wasn't able to find any related information in web searches. Maybe my understanding is fundamentally wrong, in which case I ask you to help me out...

Thank you. Leo

Answer 1

The idea is quite simple... Add the right amount of plastic.

Here is a simple diagram of a reflective measure.

You have a light source (L) and some rays hit your gray target (C). This absorbs some rays and reflects 18% of the light to the lightmeter which has a hole (B) and a sensor (A)

When you move the plastic dome, position the lightmeter in the place of the gray card, and point it toward the light, the plastic dome reflects some light, disperses some other light which does not enter the hole (B) and, as if it is well designed, will spill the equivalent of 18% of the light it received inside the hole into the sensor.

You can make the dome bigger, smaller add more plastic, or remove the thickness of the dome so the right amount of light enters and gives you a consistent reading, either incident or reflected.

It does not need a switch, only more or less plastic.

Answer 2

You used the word "assume" quite a bit. Unfortunately, your assumptions about the assumptions made by the light meter are incorrect.

You are correct that the light meter is calibrated to 18% gray. But no, there is no "multiplication by .18" going on. Don't get hung up on the number 18% or 0.18. It's just a thing, a standard. Might as well call it "nominal gray", "middle gray", or even "arbitrary gray" for purposes of using the light meter.

A reflective light meter assumes a nominal reflectivity of the subject. But different objects and surfaces reflect light differently, including differing colors, which affect the light intensity entering the meter. Subjects darker than middle gray reflect less light, and thus will be metered (exposed) higher. Conversely, subjects brighter than middle gray will be metered darker. Smooth reflective surfaces, even dark ones, with specular light reflection will show up very bright. The meter doesn't know this.

Measuring incident light makes no assumptions about the subject, because it's not measuring the subject. It's measuring the environmental light falling on the subject.

In other words, assuming equal lighting conditions and an 18% gray card as a subject, in order for an incident and a reflective light metering to yield equal exposure settings, am I correct in assuming that the only difference of both modes is that in incident mode, a light meter needs to account for an assumed reflection of 18%, but otherwise works the same way as in reflective mode?

Well, of course you're assuming equal lighting conditions, because that's the scene and light you're measuring. But no, you're not correct in assuming. By spot metering the gray card, your exposure settings will render the gray card as middle gray. That's it.

The goal isn't to get an incident light meter and reflected meter to yield equal exposure settings. They are not interchangeable or "convertible" techniques.

• Incident meters measure the light coming into the scene, from the perspective of the subject.
• Reflective meters measure the light coming into the camera from the subject, from the perspective of the camera.

An incident light meter essentially tells you "you're in Sunny 16 conditions", or "you're in indoor lighting conditions". It tells you the intensity of light, in EV.

A reflected metering tells you how much you need to compensate your exposure based on the specific subject you're shooting, because it may be backlit, or have a dark background, or have other specific conditions that make the scene lighting particular, unique, and well, not boring gray.

See also:

• Why should we be interested at incident metering?
• What is the difference between luminance and illuminance?

Answer 3

The diffusion dome itself reduces the incident light transmission to equal ~18% grey reflectance. The meter sensor inside simply reacts to the intensity of the light that falls on it, reflected or incident. I.e. there are no "switches" required and the sensor doesn't need to know what the operating state is. Use the meter wrong and it will give you bad information... it doesn't know or care.

You can get adaptor disks that will do the same thing for your camera making it suitable for making incidence readings. This is just one version.

Answer 4

The first practical light meter was a thermometer. Researchers in England circa 1900’s, took temperature readings in sunlight and in shadow, a chart was consulted which returned the camera settings. Carts and tables gave hit or miss settings. The electric light meter arrived on the scene in the early 1930’s. These light meters averaged the scene’s brightness. Kodak laboratories discovered that one could take a close-up reading of a yellow film box. Seems this box, when properly exposed was rendered as a middle-gray tone on the finished film/print.

This discovery led to the use of a target placard with an 18% reflective surface. We call this a “gray card”. The 18% gray card became a useful tool for exposure determination. An alternative to carrying a gray card was a translucent cover that mounted over the entrance of the light meter. This device furnished a light meter reading that matched a reflective 18% gray card reading.

In this method, the meter is pointed, not at the subject but in a direction that reads the light that is about to play on the subject. The method is called an “incident” light reading. Incident is old French for about to happen.

The theory of these methods, if you can figure out the exposure setting that will render one tone of the scene correct, the rest fall in line by law. Thus, rendering the 18% “middle gray” correct, provides the best basis for exposure.

Let me add, Cine photographers (movie camera operators) relied almost exclusively on the incident method. Considering the cost expended on a Hollowood movie shoot, they were under pressure to get the exposure right the first time.

Today, our cameras use sophisticated metering technology coupled with computer chip logic, all this and the experience of the photographer, get the job done.