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 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.

I don't know what the major differences are between this and the TD-404, except that the TD-404 was a Model TD-102 with a digital readout instead of analog. According to the manual this TD-504 was an all-new design. One big difference is that this model had the ability to output data to a "remote print-out device."