The absorbing filter  is an optical element employed for isolating regions of a spectrum. In general, the thicker the absorbing filter material, the more wavelengths it will absorb. However, most optical filter products ignore light diffusion and are made with a constant thickness. While the non-collimated beams pass through the filter, the optical paths vary with incident angles. Thus, the absorption difference happens and leads to the poor uniformity of transmission spectrum. In our work, a filter lens was developed to achieve the similar function of interference filter and ND filter with better spectrum uniformity. It is mounted onto a designed macro lens and supplies it with a good spectrum aberration correction. The shape of the filter lens is designed to eliminate the optical path differences between the light beams in the medium. The macro lens is made of neutral glass and shaped into symmetrical biconvex for achieving macro imaging. The spectrum characteristic of the filter lens depends on the material of the absorbing filter. In the experiment, the filter lens was prepared. The experimental results show that the spectrum uniformity of the filter lens is better than that of the normal filter.
The skin illuminated of two lights at different wavelength can be applied to detect the oxygen saturation of human blood. Due to the absorption coefficient of oxy- (HbO<sub>2</sub>) and deoxy- (Hb) hemoglobin are different at the wavelength 660 nm and 890 nm, the transmitted and reflected light within the skin can be used to compute the oxygen saturation image of skin. However, the intensities of skin images illuminated by a 20 mW NIR-LED are too low to determine the position of blood vessel when acquired by the color CCD camera. In order to improve the disadvantages, a mono camera was used and the irradiated distance and angle between LED light and test hand were adjusted to acquire the higher resolution and contrast blood vessel images for the oxygen saturation calculation. In the experiment, we developed the suitable angle to irradiate NIR light is at 75 degrees because the reflected and scattered effect could be generated significantly from both vertical and horizontal direction. In addition, the best contrast vessel images can be obtained when the shutter time is set at 44.030 ms and the irradiated distance was at the range 140-160 mm due to the intensity ratio between tissue and vessel region is the highest and the intensities of image would not be saturated or become too low when these two parameters were adjusted slightly. In future, the proposed parameters and results can be applied to the oxygen saturation measurement in the clinical diagnosis.
The yellow-ring (YR) is a chromatism phenomenon which is caused by the inhomogeneous phosphor layer of the whitelight
LED (WLED) and can be observed from the projected WLED lightspot. In general, the lights emitted from a
WLED will focus on the specific range to form a circular lightspot; in the meanwhile, the YR will appear on the
periphery of this lightspot. In our previous study, the evaluation of YR phenomenon was graded by the YR index (YRI),
which is the product of the yellow light intensity (Y) and total light intensity (I). Therein, the maximum value of YRI is
a crucial optical parameter for determining the YR degree and a YR evaluation model is as the criterion for quality
control of WLED. In this article, the YR distribution of MR-16 triplet lens module related to the distance (pitch) between
two of three WLEDs will be discussed. Experimental results show that the YRI of triple lens WLED module is lower
than one of single lens WLED module due to the overlapped effect of WLED lightspots. In addition, the YR degree of
triplet lens WLED module will be lower if the pitch is longer, but the unintended dark-corner effect will appear when the
pitch is enough longer.