Low-cost photometric testing of automotive headlamps from near-field measurement

Testing extended sources is difficult to manage because most have complex and irregular energy patterns in the near-field region, but proper performance actually takes place farther away, in the far field. In the case of automotive headlamps, performance is tested at a number of points on a flat screen positioned at a distance of 25m. Measurements are usually made in photometric tunnels. These are expensive installations to rent and usually distant from the headlamp manufacturer, who must nevertheless use them to validate new designs. The situation has grown worse in recent years as new geometries, materials, and light have come to market, leaving previous experience to count for little.

We have developed a low-cost system, shown in Figure 1, that samples the energy in each direction in the near field, then propagates it to the far-field region, onto a plane at 25m. The components consist of a commercial 8-bit CCD camera, equipped with filters, which scans the region with relevant energy values close to the headlamp, an electronic shutter to extend the available dynamic range, and propagation software to superimpose different energy patches in the final plane.¹

It is well known that incoming light to a CCD camera with its lens focused to infinity behaves such that each pixel collects the rays with a given spatial slope. This principle is used in optical metrology for some deflectometric arrangements.² In combination, the distance of the pixel from the optical axis and the focal length of the lens allow measurement of the 2D slope (u, v) of the ray corresponding to each pixel (see Figure 2). The grey level E registered at that pixel measures the energy traveling in that direction. Finally, the position of the camera at the moment of registration fixes the position (x, y) of the incoming ray. Thus, with each camera shot, we were collecting (x, y, u, v, E) enough data to locate E in the final plane.

Properly converting propagated grey level values from an 8bit camera to photometric units required several arrangements.³ A combination of filters, including equalizing and photopic, yielded the required photopic spectral response, and additional neutral filters accommodated camera response to the high luminance of the headlamp output. Vignetting effects in the optical system were modeled to include them in the calculations. Finally, an external shutter that selected optimum exposure time extended the dynamic range of the 8bit camera. All these factors ensured optimal sampling of the near field close to the headlamp.

The grey-level values, propagated by simple projection to the final plane, were then converted into real photometric values, and a calibration model developed for this purpose⁴ divided the measurement chain into the main loss and conversion factors. A global conversion constant made it possible to quantify the effect of the various parameters in the experimental setup, and results predicted by our unit compared nicely with those from a photometric tunnel for a number of different headlamps and distributions, as indicated in Figure 3.

Thus, a simple optoelectronic system built of low-cost components can properly characterize the behavior of a complex extended source such as a commercial automotive headlamp. The working principle is general enough that it can be applied to all types of extended sources.