Aims. We describe the design, operation, and first results of a photometric calibration project, called DICE (Direct Illumination Calibration Experiment), aiming at achieving precise instrumental calibration of optical telescopes. The heart of DICE is an illumination device composed of 24 narrow-spectrum, high-intensity, light-emitting diodes (LED) chosen to cover the ultraviolet-to-near-infrared spectral range. It implements a point-like source placed at a finite distance from the telescope entrance pupil, yielding a flat field illumination that covers the entire field of view of the imager. The purpose of this system is to perform a lightweight routine monitoring of the imager passbands with a precision better than 5 per-mil on the relative passband normalisations and about 3 Å on the filter cutoff positions. Methods. Prior to installation, the light source is calibrated on a spectrophotometric bench. As our fundamental metrology standard, we use a photodiode calibrated at the National Institute of Standards and Technology (NIST). The radiant intensity of each beam is mapped, and spectra are measured for each LED. All measurements are conducted at temperatures ranging from 0 • C to 25 • C in order to study the temperature dependence of the system. The photometric and spectroscopic measurements are combined into a model that predicts the spectral intensity of the source as a function of temperature. Results. We find that the calibration beams are stable at the 10 −4 level-after taking the slight temperature dependence of the LED emission properties into account. We show that the spectral intensity of the source can be characterised with a precision of 3 Å in wavelength, depending on how accurately we are able to calibrate the wavelength response of the mononochromator. In flux, we reach an accuracy of about 0.2−0.5% depending on how we understand the off-diagonal terms of the error budget affecting the calibration of the NIST photodiode. We describe how with a routine < ∼ 60-mn calibration program, the apparatus is able to constrain the imager passbands at the targeted precision levels.