Devices capable of measuring the reflectance of the skin from one location are avail-able, too. They measure either the colour of the reflected light as tri-stimulus values, or specific index-values, such as the pigmentation index (PI) or erythema index (EI).
The Chromameter CR 200 (Minolta) measures the tri-stimulus reflectance values of a target, such as human skin, illuminated with a wideband xenon arc lamp. The re-flectance values provided by the Chromameter are based on the average rere-flectance from a circular area, of diameter about 8 mm. The tri-stimulus values are converted to the CIE L*a*b* coordinates. The a* parameter is found to correlate with erythema whereas the luminance parameter, L, and the b* parameter correlate with pigmentation.
(Claryset al.2000)
The Mexameter MX-16 (Courage-Khazaka) is a device specifically made for measur-ing the melanin index, MI, and the erythema index, EI. The light source consists of twelve LEDs, whose wavelengths areλg=568 nm,λr=660 nm,λnir=880 nm. The device measures the reflectance of the skin from a circular area, having a diameter of about 5 mm. (Claryset al.2000)
The DermaSpectrometer (Cortex Technology) uses only two wavelengths,λg=568 nm andλg=655 nm. The device measures the reclectance from skin, averaging a circular area, the diameter of which is about 6 mm. (Claryset al.2000)
The EMM-01 is a new device for estimating the pigmentation and melanin indices. It uses twelve LEDs for illumination and one photodiode for detection. The wavelengths of the LEDs are λg = 560 nm, λr = 650 nm, and λnir =710 nm. The placing of the illumination LEDs and the detector is designed to reject specular reflections. The erythema and pigmentation indices are calculated as follows:
M=k·A(λr)−A(λnir) λnir−λr
(64)
E=100
A(λg)−A(λr)−M
k (λr−λg)
, (65)
whereM is the pigmentation index, E is the erythema index and A(λ) is absorption.
(Dolotovet al.2004)
5 SPECTROCUTOMETRY
In this section, two prototypes of the Spectrocutometer hardware are introduced. The light transport models and algorithms for solving the optical properties of the tissues and for calculating clinical indices is explained. Finally, the section presents two clini-cal studies made using the Spectrocutometer prototypes to asses the healing of wounds and the properties of matured scars. The description of the studies shows how the dig-ital planimetry, and chromophore mapping would work in practise, how the acquired clinical parameters relate to current medical practises, and what is the clinical signifi-cance of these measurements. The second study includes also some statistical analysis to find out which scar parameters have highest clinical value, and how do these differ-ent properties depend on each other.
5.1 Spectrocutometer prototype
Several pilot studies have been already made to study how the digital planimetry and chromophore mapping would work in practise. For these studies, a prototype of the hardware of the Spectrocutometry was needed. A schematic diagram of the second generation prototype is shown in Figure 18.
The main part of the prototype is the protecting dome, which has tree purposes 1) to act as a support where the camera and the lighting unit are connected, 2) to protect the target from ambient light, 3) to keep the angle and the distance of the camera standard for all images.
The protection for ambient light is essential because it allows the calibration of the system by imaging a known target, usually a white reference. When the image of the actual target is acquired, the spectral characteristics of the camera and the light source and the inevitably uneven light field are compensated as shown in Equation (54). The digital camera in this prototype is Canon Powershot G5 (Canon Inc, Tokyo, Japan), which is modified by removing the IR-cut filter from the sensor in order to extend the range of the camera to NIR-range. The illumination unit consist of 72 moderate power surface mounted LEDs with large illumination angles. The center wavelengths of the LEDs are 468, 520, 639, and 939 nm and bandwidths (FWHM) are correspondingly 33, 39, 19, and 81 nm. The diameter of the support unit is 83 mm, and the distance between the illumination unit and the target is 40 mm.
Figure 18. Prototype of the hardware of the Spectrocutometer: The central part of the prototype is the protecting dome, which both protects the target from am-bient light and acts as a support for camera and the lighting system keeping the imaging distance and angle constant for every shot. The lighting system consists of LED:s and a control unit, which is not shown in the figure.
The third prototype is based on a digital single lens reflex camera (DSLR), Fuji IS Pro (Fujifilm corporation,Tokyo, Japan), which is already capable of imaging through UV-VIS-NIR range as far as approximately 1000 nm. In this case the protecting dome has the shape of a cuboid of height 13 cm, width 13 cm, and length 20 cm. The size of the imaged area is thus 20x13 cm. The illumination is similar to that of the cylinder shaped prototype. This prototype is shown in Figure 19. The illumination is more even in this prototype than in the previous prototype but the light comes further away from the optical axis and shadow formation is stronger.
In both cases the images are transferred from the camera to the computer in raw format to retain the full resolution of the sensor, which is 14 bits/colour in Fuji and 12 bits/-colour in Canon. The calibration and other image processing was calculated in 16-bit format to avoid rounding errors. The accuracy of the reflectance measurements using Spectrocutometry prototype is comparable to the accuracy of a spectrometer, using in-tegrating sphere (Publication V). In both cases, the specular reflections are somewhat disturbing, and in the future the cross-polarising filters are planned to be used as ex-plained in Subsection (2.1.5).
LEDs
Chamber
Figure 19. The third prototype of the hardware of the Spectrocutometer: The protect-ing dome is now cuboid shape, and the imaged area is larger.