A method for determining or predicting a value indicative of an amount of a substance or contaminant in or on each of a series of illuminated products emitting differing levels of light intensity when illuminated, the method comprising for each product, the steps of: receiving a first signal indicative of intensity of light emitted from the product across a first waveband, said first waveband containing one or more wavelengths of intensity associated with fluorescent properties of the substance or contaminant to be detected and containing a wider band of other wavelengths emitted from the product, receiving a second signal indicative of intensity of light emitted from the product across a second waveband, said second waveband narrower than the first waveband, said second waveband within the first waveband, and the second waveband containing one or more wavelengths of intensity associated with fluorescent properties of the substance or contaminant to be detected, determining or predicting a value indicative of an amount of the substance or contaminant in or on the product, from a strength of the first signal and a strength of the second signal and from predetermined information relating to fluorescence behaviour of a same or similar product or a main substance of the product isolated from the substance or contaminant, hereinafter referred to as isolated product, in said first and second wavebands and/or relating to the fluorescence behaviour of the substance or contaminant isolated from the product in said first and second wavebands, by separating from the first signal a portion of signal strength relating to the substance or contaminant based on the second signal strength, the fluorescent behaviour of the substance or contaminant in isolation from the product, and the fluorescent behaviour of the isolated product wherein each product is a meat sample or carcass, and the main substance of the product is any of or any combination of collagen, tissue and fat, and the substance(s) or contaminant(s) to be determined is any of or any combination of fecal residue, fat, porphyrin, collagen and bacteria. receiving a first signal indicative of intensity of light emitted from the product across a first waveband, said first waveband containing one or more wavelengths of intensity associated with fluorescent properties of the substance or contaminant to be detected and containing a wider band of other wavelengths emitted from the product, receiving a second signal indicative of intensity of light emitted from the product across a second waveband, said second waveband narrower than the first waveband, said second waveband within the first waveband, and the second waveband containing one or more wavelengths of intensity associated with fluorescent properties of the substance or contaminant to be detected, determining or predicting a value indicative of an amount of the substance or contaminant in or on the product, from a strength of the first signal and a strength of the second signal and from predetermined information relating to fluorescence behaviour of a same or similar product or a main substance of the product isolated from the substance or contaminant, hereinafter referred to as isolated product, in said first and second wavebands and/or relating to the fluorescence behaviour of the substance or contaminant isolated from the product in said first and second wavebands, by separating from the first signal a portion of signal strength relating to the substance or contaminant based on the second signal strength, the fluorescent behaviour of the substance or contaminant in isolation from the product, and the fluorescent behaviour of the isolated product wherein each product is a meat sample or carcass, and the main substance of the product is any of or any combination of collagen, tissue and fat, and the substance(s) or contaminant(s) to be determined is any of or any combination of fecal residue, fat, porphyrin, collagen and bacteria.