IDS peak 2.8.0 / uEye+ firmware 3.33

Micro lenses improve the fill factor

The first purpose of micro lenses is to improve the fill factor of sensors. The fill factor is the percentage of the pixel's area that is light-sensitive and thus collecting photons. With the ideal fill factor of 100 %, the complete sensor area would be light sensitive. Real sensors have a fill factor of approx. 30-50 % depending on their technology. Other elements, like electronics, need space on the sensor area, as well. Using micro lenses can compensate that and reach an effective fill factor of more than 90 %. They collect the light that would otherwise fall onto a non-sensitive part of the photocell and re-direct it into the light-sensitive part. That is the reason, why every current image sensor has micro lenses.

Fig. 4: Using micro lenses to increase the effective fill factor

Micro lenses with CRA correction

The second (optional) purpose of micro lenses is to compensate for light rays hitting the sensor surface with an angle other than 0° from the surface normal. This effect originates from the lens' optical characteristics and is called the Chief Ray Angle (CRA). For standard lenses, only the light in the center of the lens hits the sensor surface with perfectly 0°. The more off-centered, the larger the CRA. Typically, lenses with shorter focal lengths have larger CRA.

Fig. 5: CRA on outer corners

With increasing CRA of the lens, the image can show a vignetting effect.

Fig. 6: Micro lenses without CRA correction

Fig. 7: Image captured without CRA correction shows vignetting (shading)

Some sensors have shifted micro lenses to compensate this effect. Their CRA correction is specified in degrees and refers to the CRA in the outer corners of the sensor.

Fig. 8: Micro lenses with CRA correction

Fig. 9: Image captured with CRA correction

Fig. 10: Micro lenses without and with CRA correction

Refer to the respective data sheets for more information on a specific camera model.