Light meter calibration

I've been thinking a lot lately about how to correctly calibrate my various light meters. I have several, of both the incident/lumisphere and reflective/spot types, and they're all over the place. Not only do I want them to match, I'd also like them to all give the "correct" reading based on standards and specifications.

The Internet is chock full of people sharing their rules-of-thumb and get-it-in-the-ballpark methods, but pretty much none of those even vaguely resemble what a professional might do. The closest you'll ever get to that, is recommendations for specific labs that can do calibration.

What no one ever talks about, is how those labs actually do it.

That being said, I've recently decided to tackle the problem myself. While I'm never going to match the setup of a full-blown metrology lab, I should at least be able to attempt a somewhat calculated approach.

 

As a reference, I've been working from the Exposure Metering (by Jeff Conrad) document. This document does a good job at laying out all the formulas needed to go from an actual light reading, to what a photographic light meter should show you. It appears to be somewhat based on formulas in ISO 2720:1974 (the official spec on this thing), but distills it down to what I actually want to know.

As tools, I've been using the following:

• An incident lux meter with proper NIST-traceable lab calibration (I have an Extech LT300 and a Minolta CL-200A, though they basically read the same under the same test conditions.)
• A Sekonic Exposure Profile Target II, which contains a very large and high quality gray card. I've measured this on my densitometer to have a density of 0.75, which translates to a reflectance of 17.78% (a.k.a. "18% gray")
• A big light source, which in my case is the sun under an open sky. Ideally you should use a controlled light source with a color temperature of 4700K, but I don't have anything suitable. (The smaller the light source, the bigger the variations your geometric conditions may introduce with lumisphere-style incident meters.)
• A collection of incident and reflective light meters to compare

I've decided to do this entire exercise in EV at ISO 100, since it is a standard absolute unit most light meters can be set to read in terms of. There's a lot less room for "rounding error" than something as coarse as standard f-stops or shutter speed stops.

If you want to convert a simple f-stop/shutter reading to EV, this is the formula:

EV = log2(N2/S)​

Where N is the f-stop (e.g. 5.6), and S is the shutter speed in seconds (e.g. 1/60).

The goal is, given a lux reading from a lux meter, to determine:

• What EV an incident-light meter should give at the same spot
• What EV a reflected-light meter should give from my gray card placed at that same spot

 

That all being said, here's the math I've come up with for incident-light meters:

EV100 = log2((Lux * 100) / C)​

Where C=340 for Sekonic lumisphere-style incident meters.

For reflected-light (e.g. spot) meters, it gets a little bit more complicated:

Z = 10(-1 * D)
Ls = (Z * Lux) / Pi
EV100 = log2((Ls * 100) / K)​

Where:

• D = measured density of the gray card being used
• Z = reflectance of the gray card (can skip the calculation and set directly, if you already know it and/or don't have a densitometer)
• Lux = the same incident lux reading used in the previous calculation
• K = a constant that's typically 12.5 for Sekonic/Canon/Nikon meters, and 14 for Minolta/Pentax meters

In a worked example, I get the following results:

• Measured lux: 43400
• Sekonic lumisphere incident meters should measure: EV=13.6
• Sekonic spot meters should measure: EV=14.3
• Minolta/Pentax spot meters should measure: EV=14.1

The easiest way to do this whole process is to simply make a spreadsheet, program a calculator, or write an app. (I've done the first two for my own purposes.)

 

Some quick takeaway notes:

• Make sure you actually know the reflectance of your gray card, and don't make any assumptions. While a good gray card like my Sekonic really is close enough to 18%, not all of them are.  For example, my field-friendly Lastolite Ezybalance measures at only 14% (despite claiming 18% in the specs).
• There should be a difference of approximately half a stop between an incident reading and a spot reading off an 18% gray card.  This is because meters aren't actually calibrated to 18% gray, but rather something closer to 12%. This article gives a decent overview: Meters Don't See 18% Gray