Measuring Illumination
What is Illuminance?
Illuminance is a measure of the quantity of light falling on a surface defined as the density of the luminous flux incident on a surface. Illuminance is the new term of art, illumination is the older term. One footcandle is the illuminance at a point on a surface which is one foot from, and perpendicular to, a uniform point source of one candela. So what is a candela? The light from a common candle has a luminous intensity of roughly one candela (we could get more technical, but we won’t). One lux is the illuminance 1 meter from the source. A light source that uniformly radiates one candela in all directions radiates a total of 4*Pi lumens. One lumen uniformly distributed over one square foot of surface provides an illumination of 1 footcandle.
English system: 1 footcandle = 1 lumen / square ft.
fc = candela / distance square (ft.)
fc = lumens / area (sq. ft.)
Conversion: fc = lux x .0929
Metric system: 1 lux = 1 lumen / square mete
lux = candela / distance square (m)
lux = lamp lumens / area (sq. m)
Conversion: lux = fc x 10.76
How does this relate to the real world? Good thing we don’t have to do a lot of math when we’re taking pictures or shooting video, but it sure helps to know how many footcandles we can count on from light sources and how many footcandles we need for proper exposure. Light bulbs typically are rated in terms of lumens, but since that’s a measure of light radiating in all directions, it’s not very useful in video production. What we really care about is the actual footcandles falling on our subject and/or scenery. Most professional equipment will show photometric data in terms of lux of fc at a variety of useful distances as well as the beam angle (the angle of the beam at which point the edge of the beam has an intensity 1/2 that of the center). Here’s an example of a typical chart :
In interior workplaces or homes, illuminance levels range between 10 and 100 fc on the average. In sunlit exteriors, levels may range from 100 fc in the shade to 10,000 fc or more with direct sunlight. A typical small light providing 40 fc at 3 ft. is going to make a big different when shooting indoors, but will be like pissing in the ocean when used outdoors.
How can we measure fc (or lux) without a meter that offers direct reading of fc (or lux)?
If we know we need a certain amount of light to shoot with our video camera, we can determine our lighting needs depending on the environment. It’s handy having a light meter that provides direct reading of fc or lux, however, if your light meter does not provide direct reading of fc or lux, you can figure out the fc using the following technique:
If you using a reflected light meter or still camera with a built in meter, you can figure out the fc of illumination falling on a white sheet of paper using the built in meter of a still camera by placing a sheet of white paper at a 45 degree angle to the source and 45 degree angle to your camera and set the light meter to E.I. 100 and 1/15 second, you can now take a reading and convert the meter’s f/stop reading to footcandles using the chart below:
f/1.4 = 1.25 footcandles
f/2 = 2.5 footcandles
f/2.8 = 5 footcandles
f/4 = 10 footcandles
f/5.6 = 20 footcandles
f/8 = 40 footcandles
f/11 = 80 footcandles
f/16 = 160 footcandles
f/22 = 320 footcandles
If you prefer lux, recall that lux = fc x 10.76
If you using an incident light meter, it’s a little different. Use a flat disk for a more conservative “direct” reading. Set the E.I. to 100 and the shutter speed to 1/4. Pointing the meter to the light source (in the same position of the subject, remember the intensity of the light falls off quickly as you change the subject-source distance) take your reading, you can now convert the meter’s f/stop reading to footcandles using the chart below:
f/1.4 = 1.875 footcandles
f/2 = 3.75 footcandles
f/2.8 = 7.5 footcandles
f/4 = 15 footcandles
f/5.6 = 30 footcandles
f/8 = 60 footcandles
f/11 = 120 footcandles
f/16 = 240 footcandles
f/22 = 480 footcandles
If you prefer lux, recall that lux = fc x 10.76
This use of a meter without fc readings to get fc readings technique has been derived through some experimentation using my incident meter which does have a direct fc reading so I can take a reading, get an f/stop, and then switch over to fc. The translation to using a camera meter was done with my D-SLR and using the incident reading as the reference point. The difference between the two approaches is that meters are calibrated to 18% grey, so using a white card provides different results, so you have to compensate, however, its easier finding white paper than an 18% grey card.
How does this compare to your experience? Please contact me and let me know!– David.
Footnote. An incident meter with fc readings is handy, I use a Minolta Flash Meter V incident meter that optionally gives fc readings. Once I know my exposure target (these days I’m often shooting at f/2.8 with no gain on a Panasonic HPX170 which means my key should provide 40fc or so on the subject) so I know that if I’m lighting the background of the scene that I want to be about one stop under middle, then I light it so it as 20fc falling on it and I can rough it in without having to look at the monitor. Of course I’ll look at the monitor for final tweaks, but to rough in lights, it’s faster to work with a meter than having to look at the monitor all the time.
