Impact of Lens Chief Ray Angle on Image QualityBy Alex Ning,PhD1.IntroductionLens chief ray angle refers to the angle of incidence of an offaxis ray passing through the center of thelens stop on the image plane.If the imaging medium is a silver-halide film,the chief ray angle wouldnot make too much difference to the final image quality because the film grain has isotropic angularresponse.However,if the imaging medium is a CCD or CMOS imager,the lens chief ray hassignificant impact on the final image quality.This is because the basic imaging element (a pixel)ofCCD/CMOS imagers does not have isotropic response.This paper exams the effect of lens chief rayangle on the overall image quality.2.ExperimentThe Micron MT9D011 imager is used with two different lenses:Sunex PN DSL746 with a 1G2Pstructure and a competitor lens (also a 1G2P lens).Two lenses have the same f-stop of 2.8.The Sunexlens is designed to limit the max.chiefray angle within 20 degrees at the corner of the imager.Themax.chief ray angle of the competitor lens is 25 degrees.An uniform white background is captured byusing both lenses.The result is shown as follows.Sunex DSL746Competitor LensAs can be seen from the above figure,the DSL746 lens has more uniform response in terms of colorand corner brightness.The ratio of corner brightness to the center is known as "relative illumination".It is also clear that the white balance(ratios between G,B and R)is more consistent from center to thecorner with DSL746 lens.This difference can be explained in terms of chiefray angle mis-match withthe sensor.At off-axis field angles,the higher chief ray angle of the competitor lens is outside the peakresponse region of pixel micro-lens.This results in a reduction in the overall response at off-axis angles.With color imager the higher chiefray angle can also cause cross-talk between adjacent pixels ofdifferent color.The cross-talk causes the ratios between R,G and B to vary from center to corners.Atypical result is that the center will tend to be warmer,and corners cooler,as shown in the above figure.Page 13.Relative lllumination and Relative ColorThe above described phenomenon can be measured using many commercially available programs suchas PhotoShop and Jasc Paint Shop Pro.We would first read out the pixel values for each R,G and Bchannel in the center of the image.These values will be used as baseline numbers.We will then readout the R,G and B pixel values at each of the four corners.Relative illumination (RD can be computedas the ratio of corner luminance Y (=0.3*R+0.59*G+0.11*B)to the center luminance.The whitebalance is represented by the ratio ofR to B.If R/B >1,the image is warm.If R/B<1,the image iscool.Relative color(RC)is represented by the corner R/B ratio divided by the center R/B ratio.CenterR:150CenterG:150R150B:149G:131B:115Average ofAverage of4 corners4 comersR84R:99G:101G:92B:90B:101Center-R/B=150/149=100.6号Center-R/B=150/115=130.4号Corner-R/B=99/101=98%Corner-R/B=99/101=93.3号Relative Color(RC)=(Corner-R/B)/(Center-Relative Color(RC)=(Corner-R/B)/(Center-R/B)=97.4号R/B)=71.6%Center Y=0.3*R+0.59*G+0.11*B=149.89Center Y=0.3*R+0.59*G+0.11*B=134.94Corner Y=0.3*R+0.59*G+0.11*B=100.4Corner Y=0.3*R+0.59*G+0.11*B=89.38Relative illumination (RI)=678Relative illumination (RI)=6684.ConclusionMobile imaging modules demand very short profile lenses.The short profile lenses tend to have veryhigh chief ray angles.The high chiefray angle can be detrimental to the image quality given the limitedangular acceptance of the micro-lens array on the imager.This paper demonstrates that a 5 degdifference in the chief ray angle can cause a significant relative color issue (only 71.6%of the idealgoal)on the Micron 1.3MP imager.An acceptable relative color value should be >95%but less than105%for a good imaging system.Page 2