Incident Meter

Overview

An incident meter (German: Belichtungsmesser für Lichtmessung) is a handheld light meter that measures the intensity of the light falling on the subject – not the light reflected from the subject. Characteristic is the white, hemispherical diffuser dome (Diffusorkuppel, called "Lumisphere" by Sekonic) over the sensor.

The crucial difference compared to measurement with the camera's light meter or a spot meter (both reflection measurement): The incident light measurement is independent of the subject's brightness, color, and reflectivity. A black suit and a white wall, illuminated equally, will yield the same reading. This makes the method particularly suitable for controlled lighting on set, where the amount of light is deliberately set by the luminaires.

Principle of Operation

For measurement, the device is held at the subject's position, with the dome typically pointed towards the camera. The hemispherical dome integrates the light falling from various directions and provides a cosine-weighted evaluation of the light incident on a three-dimensional surface.

On many devices, the dome can be retracted or replaced with a flat diffuser. In this flatter position, the device measures more directionally, which facilitates the precise measurement of individual light sources and the determination of lighting ratios – for example, between key and fill light.

Operation and Output Values

The recording parameters are first set on the device, from which the exposure recommendation is derived:

• Inputs: Sensitivity (ISO/ASA) and – in film/TV operation – the frame rate or the derived shutter angle/exposure time.
• Output: the correct aperture. Cinema-compatible models output this as a T-stop (transmission aperture of cine lenses) instead of an F-stop.

Use on Set

In film and TV lighting, the incident meter is used by the Director of Photography or the Gaffer to set light levels precisely and repeatably: bringing the key light to the desired working aperture, setting fill and effect lights to the desired ratio, and ensuring consistent exposure across multiple setups. Even when working with electronic cameras, incident light measurement remains relevant – supplementing the waveform monitor, false color, and histogram – because it quantifies light values in absolute aperture values, independent of the subject.

Common manufacturers of light meters with incident light measurement capability include Sekonic and Kenko/Minolta. Many devices combine incident light measurement (dome) and reflection measurement (spot attachment) in a combination model.