Method for determining shadowline location on a photosensitive array and critical angle refractometer employing the method

The present invention relates generally to methods for determining the location of a transitional boundary or "shadowline" between an illuminated region and a dark region on a linear scanned array of photosensitive cells, such methods being particularly applicable to critical angle refractometers wherein shadowline location is correlated to index of refraction of a test sample. A method for determining a cell crossing number of a shadowline between illuminated and dark regions of a linear scanned array of photosensitive cells, and an automatic refractometer using the method, are disclosed. The array is scanned to extract a response signal from each of the photosensitive cells, and the response signals are converted from are converted from analog form to digital pixels, thus yielding a set of data points that collectively represent an illumination distribution curve over the array. A range of cells within which the shadowline resides is established by analyzing the illumination curve data. The second derivative of the illumination distribution curve over the established range of cells is calculated and the greatest positive area bounded by the second derivative is identified. The centroid of the greatest positive area is found and its cell number coordinate is deemed the cell crossing number of the shadowline. The method provides improved precision to better accommodate varying levels of illumination intensity.