This is obviously not a light meter, but I'm listing it here anyway until I find a better place. It does measure light and it is a photographer's tool, it's just a bit different in the job it performs.

A densitometer is a lab machine. They come in two types, reflection and transmission. Reflection densitometers are used on prints, transmission on flim transparancies (slides or negatives). Either way, they do the same thing. They emit a given quanity of light and measure how much of it is reflected back or passes through film. The less light that is measured means the greater the density of the spot that's being measured.

This is extremely useful if you're evaluating how an emulsion reacts to light and chemicals, and changes in each. Any zone system book that deals with darkroom work will (Ansel Adams's The Negative and The Print are great examples) will explain the theory and process in detail.

Macbeth used to make and sell quite a range of these machines. Once in awhile I find one at an electronic junk shop and pick them up. Usually they have no idea what they are. If you get one, like this transmission densitometer, you want to make sure that the #4471 photomultiplier tube (located in the snout of this thing) is good, because photomultiplier tubes are expensive to replace and may take some doing even to find out who would have them for sale.

Macbeth is gone now, sadly. X-Rite now sells their ColorChecker board, and they also had their own line of densitometers. I do not know whether they acquired Macbeth or just parts of the business.

According to the owner's manual, this TD-404 and the Macbeth TD-102 are almost identical. The big difference is that the TD-102 has an analog meter (needle and scale), whereas the TD-404 is digital. The digital readout is by nixie tubes, which are little vacuum tubes that have what look like a stack of neon tube numerals inside. If you want a great example of what a nixie looks like, check out the end of the Fort Knox segment of the movie Goldfinger, where James Bond has the bomb open and they show the countdown timer. Those are nixie tubes. I remember growing up and never thinking nixie tubes were interesting. Now that they're long gone and quite expensive, I wish I'd kept and acquired more things that had them.