UMaster description

Imaging colorimeters are all based on CCD sensors and generally color filters. The difference between the systems lays in the accuracy, the signal over noise ratio, the spatial resolution and the quality of the imaging optics. UMaster is based on a Peltier cooled CCD sensor with true 16-bit analog digital converter. Four color filters dedicated to each CCD sensor are mounted on a motorized color wheel. A second motorized wheel with flat densities is also available for an automatic adjustment to the source luminance.

High Accuracy
ELDIM is manufacturing on its own all the key components of its systems. The quality of the optics is optimum thanks to advanced technologies such as magneto-rheological polishing or stitching interferometry. Antireflective coatings and optical alignments are also performed in house to reduce straight light and parasitic polarization. UMaster uses an objective telecentric on the sensor side for imaging. The color accuracy is ensured using 4 dedicated color filters adapted to the spectral response of each CDD. UMaster uses a new technology based on electron beam evaporation that gives filters with higher transmittance and enhanced accuracy.

High Sensibility
Peltier cooled CCD sensor with true 16-bit analog digital converter and color filters with high transmittance allow optimum sensitivity for UMaster. Large size CCD versions can be used to detect very low light levels while maintaining a good spatial resolution.

High Dynamic
UMaster integrates different neutral density filters on an automated wheel. These neutral densities are spectrally flat thanks to the use of interference coatings deposited on neutral glass. Automated control of these densities is available for each color filter independently to improve the accuracy in particular for LED type sources.

A range of imaging objectives
UMaster is available with objectives of different aperture (8° and 16°) and different CCD sensor resolutions (1M to 16M pixels). Additional optics for high spatial resolution are also available.

More than just Luminance & color: Polarization
In addition to luminance and color imaging, UMaster can realize polarization imaging at different wavelengths. This option is available for all the wavelengths in the visible range (band pass 10nm). The system performs automatically seven measurements with different polarizers and wave plates from which it computes all the polarization parameters throughout the image in addition to radiance. Stokes vectors are available for each pixel of the image.

Dependence of flux with distance for standard objectives
The flux emitted by an elemental surface dS₂ of luminance L in the small angular cone 2θ₂ is given by:

We want to calculate the flux collected by and elemental surface dS₁ on the detector.

　The conservation of the geometric etendue gives the following relation between the surfaces:

Using the relation between F1, F2 and the focal length F# we finally obtain:

The signal seen by the detector is then dependent on the distance of the object. At 10F# the reduction is about 20% as shown in the figure with a big impact on the accuracy.

Flux stability for telecentric objectives
For a telecentric configuration dS₁/F²=dS₂/F₂² and M₁ is independent of the distance in first approximation. If we develop the cosines to the third term we can find:

The figure shows a flux reduction lower than 1% for 5F# distance and lower than 0.1% for 15F#. A single calibration is then sufficient for all practical distances.

Theorical dependence of the flux with the object distance (standard optics and telecentric optics)

MTF and Distorsion
The MTF (modulation transfer function) of the sensor is the modulus of the Fourier transform of the PSF (point spread function). The better the equipment is and the smaller the image of a punctual source (PSF) is which means the larger the MTF is. To say it differently the cut off frequency of the MTF needs to be as high as possible. This characteristic of the sensor is directly linked to the optical MTF of the single optics (without CCD and electronics), but also to the CCD pixel size, to the CCD quality and to the electronic chain.

At ELDIM, optical designs and production for all equipments are home made. This ensures a high and stable quality for all our optics. ELDIM’s low distortion optics ensure a perfect image with minimized optical defect compensations. Most of common equipments compensate for important distortions by software calibration. This allows redressing line images on the border of the image but degrades the global MTF and introduces artifacts in the image. Furthermore, the modulation Fourier Transform (MTF) of our optic is very large since it allows resolving very compact line patterns. On the figure hereafter we compare the resolution of our optic to commercial optics. Also important is the MTF stability ON and OFF axis on the whole field of View. It’s easy to obtain an excellent MTF in the middle of the image (ON-axis) when not taking into account the MTF on the border of the field of view (OFF axis). ELDIM’s MTF is very stable which means that patterns in the corner of the image are as resolved as patterns in the center of the image.

Additional optics for high spatial resolution
ELDIM offers now a set of additional optics to allow high and ultrahigh spatial resolution measurements of displays at the pixel level. These additional optics image the object on the sensor with a fixed magnification ratio at a fixed working distance. They offer an easy way to get accurate color images at high spatial resolution.