Handheld meter measuring light intensity on set in foot-candles or lux. Gold standard for consistent lighting setup across shooting days.
You unpack the thing, hold it up to the light, and immediately see if your light bank has enough power — or not. The densitometer is your measuring caliper for light, nothing more, nothing less. While the camera assistant juggles with light meters and spot meters, the DoP often sits next to them with a densitometer, checking the absolute brightness in the room. Measurements are taken in foot-candles (fc) or lux — depending on the scale your device has. It's basically a small photocell with a display that tells you in real-time: Here are 800 lux, there 1200. No conversion, no interpretation.
The practical benefit lies in reproducibility across shooting days. If you shoot a scene on Monday and have to reshoot on Friday — with the densitometer, you can ensure that the lighting is exactly the same. You position the device in the same spot on set where the actors stand and compare the values. This is invaluable, especially for multi-shot scenes or if you want to avoid parallax issues later. The difference from a classic exposure meter: The latter measures reflected light (dependent on surface color), while the densitometer measures the actual amount of incident light — it doesn't care if the wall is white or dark gray.
On set itself, you use it less for live camera exposure — that's what the spot meter is for. The densitometer is more your quality control instrument for the lighting setup itself. You check with it: Is the ratio between key light and fill correct? Do I really have a four-stop difference between the main light and the fill? Does the ambient brightness of the room still match the rest of the set? Some DoPs also use it for light studies before shooting — to see in prep how much power they need or if a location is salvageable at all.
Good devices are robust and last a long time, but the photocell can become sluggish over time. Regular calibration helps. And: For color temperature measurement, the densitometer is not your friend — for that, you need a color meter. But for pure intensity? This tool doesn't lie.