Notes by Dr. Optoglass: The Units of Photometry – Part II

Topics Covered:

  • Nits and Lux
  • Footcandles and footlamberts
  • Typical light levels for various scenarios

Actually, I’m not all that interested in the subject of photography. Once the picture is in the box, I’m not all that interested in what happens next. Hunters, after all, aren’t cooks – Henri Cartier-Bresson

In part I we covered measuring light at source. Now we have to find a way to measure light at destination. Destination light can be the result of many sources, and deserves its own units.

Nits, the candela way

The amount of light in candela per square meter is called Luminance. Cd/m2 is also called Nits. cd/m2 (or nits) is used when precise measurements are usually required – like in display monitors.

The light from a monitor is not meant to illuminate anything, and its area of influence is very small.

Lux, the lumens way

The amount of light in lumens per square meter is called Illuminance. lm/m2 is officially called Lux. Lux is used when we need a good idea of light falling on a given area – especially if that area is large.

The light from a bulb is meant to illuminate an area, and its area of influence is comparatively large. Practically, we don’t mind some error in its measurement. After all, we are not going to peak in every corner of a room, are we?
Lux vs Nits

To convert lux to nits, use this formula:

Lux = ? x Nits = 3.14 x Nits

If I have 10 nits, it gives me 31.4 Lux.

Footcandle, another lumens way

To confuse people, a unit similar to Lux is used where ‘feet’ is more common than ‘meters’ – the footcandle. Footcandle (fc) is lm/ft2. It is related to lux thus:

Lux = 10.764 x fc = 10 x fc

The approximation is acceptable in common practice. 10 fc will give me about 100 lux.

The Lambert and Foot-lambert

Both lambert and foot-lambert are non-SI units, and again are cross-overs from ‘feet’ land. A foot-lambert (fL) is related to nits thus:

Nits = 3.426 x fL

If I have 16 fL, I get about 55 nits.

Right, I think that’s about enough units – at least it shows how important light is! How does all this work in practice? It’s simple really – once you’ve memorized the units and their relationships, it becomes routine. Here’s how:

Table to learn units

Now you know what to use in which situation. Here are a few real world scenarios:

  • Computer monitors can have a luminance level of 80 to 300 nits.
  • HDTV monitors can have a luminance level of 500 to 1000 nits.
  • The SMPTE requires cinema screens to have a luminance level of 55 nits (16 fL).
  • The sun has a luminance of about 1.6 billion nits. This is about 10,000 fc or about 100,000-130,000 lux.
  • Typical studio lighting is about 1,000 lux.
  • Golden hour is about 400 lux.
  • Office lighting is about 300-500 lux.
  • A living room is about 50-100 lux.
  • A full moon on a clear night is about 0.27 lux.
  • A moonless clear night sky is 0.002 lux.

Now we’re beginning to get a handle on light, aren’t we? Next, we’ll convert this knowledge into camera exposure.

Takeaways:

  • The amount of light in candela per square meter is called Luminance, measured in nits.
  • The amount of light in lumens per square meter is called Illuminance, measured in lux.
  • A footcandle is lumens per square feet.
  • One footlambert is 3.426 nits.
  • Typical lux levels on earth can vary from 0 lux to 100,000 lux or more.

Links for further study:

Next: Camera Exposure
Previous: The Units of Photometry Part I