Spectral responsivity vs. action spectrum in digital photography

We propose an experimental and theoretical methodology based on spectroradiometric procedures to determine the spectral sensitivity of digital still cameras (DSC). The opto-electronic conversion spectral functions (OECSF's) are obtained from normalized digital level (NDL) vs. spectral exposure data, fitted by a typical sigmoidal transition curve. At this stage, we define spectral responsivity r(λ, H) and action spectrum a(λ, NDL), which are conceptual derivations of the general concept of spectral sensitivity of any image sensor. Then, the real spectral responsivity and action spectrum are represented as 3-dimensional functions, where r(λ, H = constant) and a(λ, NDL = constant) profiles are scaled relatively to determine the valid linear output range of the digital image capture device. This range would serve to design the colorimetric profile of the camera with CIE-1931 XYZ standard observer, which is considered as a linear color image sensor.This research was supported by the Comisión Interministerial de Ciencia y Tecnología (CICYT) (Spain) under grants TAP96-0887 and TAP99-0856

Reliability in perimetric multichannel contrast sensitivity measurements

Background: In this study, the reliability of perimetric contrast sensitivity measurements favouring the achromatic, the red-green and the blue-yellow post-receptorial mechanisms was analysed. Methods: A new technique, multichannel ATD perimetry, provides spatial and temporal stimuli favouring the detection by an achromatic mechanism (A), from a magno or parvocellular origin or by a red-green (RG) chromatic mechanism, with a parvocellular origin or a blue-yellow (BY) mechanism, with a koniocellular origin. The repeatability and reproducibility of contrast sensitivity measurements with these stimuli were studied in a group of 40 healthy subjects. The analysis was carried out on 21 testing points within a 60° by 40° fovea-centred region of the visual field. Results: The within-observer repeatability for the four mechanisms studied is either good or excellent when the intra-class correlation coefficient (ICC) can be calculated. For the remaining points, the Friedman's test finds that the measurements are repeatable. The between-observer reproducibility was either excellent or good in cases where the ICC was applied and according to the Friedman's test all results were reproducible. Conclusions: The results obtained showed good repeatability and reproducibility with A, RG and BY stimuli, although with BY stimuli repeatability is slightly worse. Future studies on the diagnostic validity of this device are based on the fact that changes of sensitivity can be compared by means of a visual single task, contrast sensitivity measurement and using a common metric.The ATD multichannel perimeter was built thanks to the support of the Spanish Ministerio de Ciencia y Tecnología Grants DPI2000-0116-P4-02 and PTR 1995-0909-OP, in collaboration with INDUSTRIAS DE OPTICA SA (San Cugat del Vallés, Spain)

La visión del movimiento (I)

La sensibilidad al movimiento es un aspecto fundamental de la visión. Por un lado analizamos el movimiento de los objetos de nuestro campo visual para poder interaccionar con ellos. Por otro, al movernos dentro de nuestro entorno, se producen también cambios en la imagen retiniana de los objetos que nos rodean aunque éstos permanezcan estáticos. En ambos casos se producen cambios espaciotemporales de luminancia en la imagen percibida y estos cambios son la fuente de información sobre la disposición del entorno y el movimiento del observador con respecto a ese entorno. El análisis de esta información permite establecer la existencia de los límites de detección del movimiento y las características diferenciales entre los movimientos reales y los movimientos aparentes

La visión del movimiento (II): fundamentos fisiológicos y modelos

El concepto de movimiento aparente, introducido en la primera lección sobre la visión de movimientos, sienta las bases para entender porque el sistema visual puede interpretar movimientos debido al muestreo temporal que realiza de la información. El problema del muestreo surge en el procesamiento debido a la comprensión de la información del objeto. En la segunda aparte de esta lección se introduce el estudio de los fundamentos fisiológicos de la visión de movimientos, un paso necesario para establecer los modelos para el análisis de movimientos

Óptica fisiológica : problemas

El presente libro presenta una amplia colección de problemas y de cuestiones de varios aspectos que constituyen la óptica fisiológica e incluye temas generales como son el estudio de la óptica geométrica aplicada al ojo, la acomodación, la presbicia, las ametropías esféricas el astigmatismo o la visión binocular u otros más específicos como la afaquia, la anisometropía y la aniseiconía. El libro se cierra con un capítulo sobre la visión cromática y la colorimetría

Óptica fisiológica : problemas

El presente libro presenta una amplia colección de problemas y de cuestiones de varios aspectos que constituyen la óptica fisiológica e incluye temas generales como son el estudio de la óptica geométrica aplicada al ojo, la acomodación, la presbicia, las ametropías esféricas el astigmatismo o la visión binocular u otros más específicos como la afaquia, la anisometropía y la aniseiconía. El libro se cierra con un capítulo sobre la visión cromática y la colorimetría

Characterization of a digital camera as an absolute tristimulus colorimeter

An algorithm is proposed for the spectral and colorimetric characterization of digital still cameras (DSC) which allows them to be used as tele-colorimeters with CIE–XYZ color output, in cd/m2. The spectral characterization consists in the calculation of the color-matching functions from the previously measured spectral sensitivities. The colorimetric characterization consists in transforming the raw RGB digital data into absolute tristimulus values CIE–XYZ (in cd/m2) under variable and unknown spectroradiometric conditions. Thus, in the first stage, a gray balance was applied over the raw RGB digital data to convert them into RGB relative colorimetric values. In the second stage, an algorithm of luminance adaptation versus lens aperture was inserted in the basic colorimetric profile. Capturing the ColorChecker chart under different light sources, and comparing the estimated XYZ data according to the developed color model in relation to the measured XYZ data (in cd/m2) using a telespectroradiometer, we verified that the proposed characterization model may be broken down into two portions. Firstly, there is the basic colorimetric profile in combination with the new luminance adaptation algorithm. Secondly, there is the linear correction term due only to the mismatch of the color matching functions of the camera. Although the linear color correction term works relatively well, despite the imposed initial conditions (unknown spectral content of the scene), the separation of the proposed characterization model into two portions (raw and corrected performance) would allow the future comparison of various commercial cameras.This research was supported by the Comisión Interministerial de Ciencia y Tecnología (CICYT) (Spain) under grant TAP99-0856 and by the Ministerio de Ciencia y Tecnología (Spain) under grant DPI2002-00118

Testing the reciprocity law in digital photography

An experimental and theoretical methodology based on spectroradiometric measures is proposed to test if the reciprocity law is verified in digital photography. Taking into account that the spectral exposure H(λ) is proportional to the spectral radiance Le(λ) of the object and the photosite integration time t of the electronic shutter time and inversely proportional to the f-number N of the zoom-lens, this radiometric law declares that identical values of spectral exposure yield identical responses even if the fnumber N and/or the exposure time t change. Historically, the photochemical materials present some deviations to this law, but it is not clear if this law holds for the image sensors of digital cameras. The test is based on the new concept of the opto-electronic spectral function (OECSF), that is, the empirical relationship between the normalized digital output level of the camera and the spectral exposure obtained by a monochromator set-up. The transition curve that fits the OECSFs in the color channels for our digital image capture device is the sigmoid function with four parameters. Varying exclusively the f-number N, the OECSFs in some irradiance scale exposure series (Le(λ) and N free, t fixed) overlap in any color channel. The same occurs with time scale exposure series (Le(λ) and t free, N fixed) or mixed scale exposure series (Le(λ), N and t free). These results indicate that this radiometric law, unlike in photochemical photography, is verified in digital photography with monochromatic light.This research was supported by the Comisión Interministerial de Ciencia y Tecnología (CICYT) (Spain) under grant TAP99-0856

Calculation of the color matching functions of digital cameras from their complete spectral sensitivities

We propose a new algorithm to obtain the color matching functions of any digital camera, with correct weights and white balance, from the relative scaling of their complete (3-D) spectral sensitivities obtained from real spectroradiometric data. Thanks to this algorithm, it is possible to predict the RGB digital levels of any digital camera in realistic illumination-scene environments — spatially non-uniform illumination field, variable chromaticity and large dynamic range of luminance levels — opening the possibility of transforming any digital camera into a tele-colorimeter. The illumination-scene test was the Macbeth Color- Checker Chart under three different light sources (halogen, metal halide and daylight fluorescent lamps), provided by a nonstandard light box. The results confirmed that it is possible to predict any RGB digital levels exclusively by varying the f – number of the camera zoom lens.This research was supported by the Comisión Interministerial de Ciencia y Tecnología (CICYT) (Spain) under grants TAP96-0887 and TAP99-0856

Estimation of the device gamut of a digital camera in raw performance using optimal color-stimuli

Using a spectroradiometric model of capture for a digital camera based on the mathematical description of the empirical opto-electronic conversion spectral functions (OECSF), the capture of MacAdam or optimal spectra with fixed illumination level is simulated. This model of capture allows to change freely the f-number of the zoom-lens and/or the photosite integration time of the electronic shutter of the camera, regardless of the spectral composition of the stimulus. If we follow the procedure employed by MacAdam in 1935 working with the CIE-1931 XYZ standard observer, these color-stimuli are arranged in decreasing pyramidal form as the luminance factor increases for any chromaticity diagram (CIE-xy, UCS-u'v' or CIE-L*a*b*). These loci are often called MacAdam limits or Rösch color solid. On the other hand, transforming the simulated RGB digital output levels of the optimal colors to XYZ data through the raw colorimetric profile with luminance adaptation of our digital image capture device, the corresponding MacAdam loci for each luminance factor are smaller than those of the colorimetric standard observer. This systematic desaturation of the optimal color-stimuli shows that our color device, in raw performance, desaturates in general the real color-stimuli, so this result justifies the additional use in digital photography of color correction algorithms, more or less complex, in order to reach the colorimetric status of color reproduction.This research was supported by the Comisión Interministerial de Ciencia y Tecnología (CICYT) (Spain) under grant TAP99-0856