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R. P. Tornow, R. Laemmer, C. Mardin; Quantitativ Imaging Using a Fundus Camera. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1206.
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© ARVO (1962-2015); The Authors (2016-present)
To improve the quality of digital color fundus images for quantitative imaging.
A Zeiss Fundus camera FF450 with a JVC KY-F75U 3-chip-CCD-camera was used for image acquisition. Uncompressed tiff-images were analysed. Two main points are important for quantitative imaging: 1) The transfer function for each CCD-sensor (R, G, B) has to be linear (Pixel_value = const x light_intensity) to perform image analysis such as arithmetic ratios, flat fielding etc. Therefore, gamma has to be set to 1 and the offset has to be zero or compensated. 2) The light intensity has to be correctly set to exploit the full dynamic range of each of the three CCD-sensors to optimize the photometric resolution. Due to the spectral distribution of the fundus reflection, the intensity distribution in the three channels in conventional digital fundus imaging is rather non-uniform (histograms of R, G, B channels, figure left): R/G/B is about 2/1/0.5 resulting in a poor photometric resolution especially of the blue channel and often saturation of the red channel. To overcome this problem, a filter was developed and introduced in the illuminating beam path of the fundus camera with only three small transmission bands in the red, green and blue spectral range. The spectral width of the transmission bands is highest for the blue range (50 nm) and lower for the green (25 nm) and red (20 nm) range to compensate for the non-uniform fundus reflection.
Fundus images taken with the developed filter and optimized settings of the CCD-camera show histograms with nearly equal distribution of intensity: R/G/B is about 1/1/1 (figure right). These images can be used for quantitative imaging and shading correction can be performed. Due to the increased photometric resolution of the blue channel and the linear transfer function, the distribution of macular pigment can be assessed from the blue and green images. This filter also decreases the light exposure at the fundus, because only light that is detected by the CCD-camera is used for illumination.
With the developed special filter and optimized settings of the CCD-camera, a fundus camera can be used for quantitative imaging allowing e.g. the assessment of the distribution of macular pigment.
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