Laboratory optical imaging systems for fluorescence and bioluminescence use a sensitive charge-coupled device (CCD) camera to produce quantitative measurements of very low light intensity, detecting signals from specimens labeled with optical fluorophores or luminescent emitters. Commercially available systems typically provide quantitative measurements of light output, in units of radiance (photons s−1 cm−2 SR−1) or intensity (photons s−1 cm−2). We describe a quality assurance system for low-light imagers, based on an LED-illuminated thin-film transistor (TFT) liquid crystal display module. The light intensity is controlled by pulse-width modulation of the backlight, producing intensity values ranging from 1×106 photons s−1 cm−2 to 4×1013 photons s−1 cm−2. The lowest light intensity values are produced by very short backlight pulses (i.e. approximately 10 μs), repeated every 300 s. This light source provides a stable, traceable intensity standard that can be used for routine quality assurance of optical imaging systems.