Improved OECF charts open a new dimension for measurements!
The upcoming revision of ISO 15739 has altered the chart layout of the original version reflected in our TE241. The patches in the center of the chart for noise measurements, low frequency structures, and the gradient will be removed due to the fact that the patches cause stray light and reflection problems. With this revision in mind we designed our OECF chart TE264. For our default versions, grey levels are generated on lithographic film by using a half toning process. This in fact causes problems with high-resolution cameras that are able to resolve the half toning structures. Therefore Image Engineering has designed the optional X-version of the TE264 on fine grain photographic material with its manually assembled grey filters on the chart. The TE264X is offered in almost any contrast level up to 1,000,000:1. The standard version provides a contrast level of 10,000:1.
Our engineers have also developed a new version of the TE264X and named it TE270X.
The TE270X chart is equipped with two polarizing filters in the center of the chart. For cameras that do not allow a manual adjustment of the exposure, the automatic exposure control (AEC) is fooled by modifying the density (transmission) of the central part of the test chart. Especially low-end cameras and camera modules in mobile devices often do not have manual exposure adjustment.
For high dynamic range cameras, the OECF charts with patches calculated according to ISO 14524 and are not that convincing. Densities are calculated according to the cube root formula and result in large steps in the high-density patches. Image Engineering's solution to the problem is the TE269, a 36-patch OECF chart which basically follows ISO 14524 but implements additional steps especially at higher density levels. This modification helps to calculate the dynamic range with a higher level of accuracy. The selection of a suitable OECF chart should in general be based on the dynamic range of the test device. The chart should have a contrast ratio that is higher than the expected dynamic range of the camera under test. Here you will find a short guide to select the suitable chart for your purpose.
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