Notes by Dr. Optoglass: How to Measure Light

Topics Covered:

  • How Light Works
  • Radiometry vs Photometry
  • What is Brightness

Light makes photography. Embrace light. Admire it. Love it. But above all, know light. Know it for all you are worth, and you will know the key to photography – George Eastman

The full spectrum of light is represented thus:

As you can see, the visible region (light) is a very small part of the whole. Even though every wavelength affects us in some way or the other, only the visible range contributes to vision, or seeing. Light is properly electromagnetic radiation (EMR) – it has electrical, magnetic and particle properties, all rolled into one.

Here, are concerned not with the colors but with the brightness range of this region, and how to properly deal with it.

How is light produced?

Most times there’s a particle reaction a photon(s) is released. Don’t disregard a photon – we already know that one rod in our retina can detect a single photon. The most common source of light, though, is thermal – in other words – heat. That explains the ‘radiation’ bit in EMR.

Every object (body) on earth emits radiation. Without going into complex details, let’s just say the intensity of radiation depends only on the Temperature of the body. Temperature in SI units is measured in Kelvin (K).

Low temperatures radiate light towards the red end of the spectrum, and high temperatures radiate light towards the blue end. Isn’t it strange that red is warm and blue is cool?

Light from the sun – the light on which our eyes have evolved – is about 6073K (Goody & Yung 1989, pp. 482, 484).

Radiometry vs Photometry

Because EMR is one thing, and light is only a part of it, there are two major ways in which visible light can be measured.

Radiometry measures EMR (and light) as radiant energy in terms of absolute power. In this world, light is all about Energy and Power (Watts).

Photometry measures light as perceived brightness relative to the human eye. In this world, we talk about Luminance or Luminosity.

Why do we need photometry? As we have seen earlier, our eye is not equally sensitive to all wavelengths. Photometry accounts for this by weighing the measured power of each wavelenth with the sensitivity of the eye at that wavelength.

Before we jump into the world of measuring light, we have to first define brightness.

What is Brightness?

When we say a light source is bright, do we mean:

  • it delivers many rays of light, or
  • it concentrates the many rays of light into a narrow beam, or
  • it is seen against a dark background or reflected?

In the first case, if a bulb emits light rays in all directions, do we measure the brightness by this absolute value? Or do we see how many rays fall into a small unit area (similar to how we measure density)? Or do we only consider light as it is reflected (light is always reflected – what we see in the world is reflected light) of shiny or matte surfaced bodies?

Which kind of measurement is most important to us? The answer is, it depends. Just as we measure mass, weight, volume and density of a body, and use the unit of measurement we need the most at that point of time, we have many ways to measure the photometric values of light. Depending on the situation we are measuring, we might find it convenient to use one unit over another in that situation.

The key is to identify the situation correctly, and then choose the unit that gets us to our goal the quickest.


  • Light is related to temperature.
  • Radiometry measures EMR (and light) as radiant energy in terms of absolute power.
  • Photometry measures light as perceived brightness relative to the human eye.
  • There are many ways to measure brightness. The key is to identify the situation correctly and then find the best unit for the job.

Links for further study:

Next: The Units of Photometry Part I
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