Notes on the determination of the angular subtense of the apparent source in laser safety

In the wavelength range of 400 nm to 1400 nm, the retinal thermal exposure or emission limits depend on the angular subtense of the apparent source (Greek symbol “alpha”). For the case that a given laser beam is associated to an extended source, according to IEC 60825-1 or ANSI Z136.1 the emission permitted for a given class (such as Class 1) can be substantially higher as compared to the case of a small source. In this paper, earlier discussions on the classification concept to analyse extended sources are summarized and commented. While it is historically justified to refer to the “apparent source” such as when the optical source is a diffusor as the classical example of an extended source, the more general understanding and terminology is to associate “alpha” with the angular subtense of the retinal image, i.e. the irradiance profile on the retina. This is particularly important when the aperture stop on the imaging system (the eye) reduces significantly the retinal image as compared to the angular subtense of the source, which is possible for coherent and partially coherent beams. In a second part of the paper, modelling results for the potential impact of the aperture stop to reduce the retinal image are discussed for the example of two partially coherent beams.In the wavelength range of 400 nm to 1400 nm, the retinal thermal exposure or emission limits depend on the angular subtense of the apparent source (Greek symbol “alpha”). For the case that a given laser beam is associated to an extended source, according to IEC 60825-1 or ANSI Z136.1 the emission permitted for a given class (such as Class 1) can be substantially higher as compared to the case of a small source. In this paper, earlier discussions on the classification concept to analyse extended sources are summarized and commented. While it is historically justified to refer to the “apparent source” such as when the optical source is a diffusor as the classical example of an extended source, the more general understanding and terminology is to associate “alpha” with the angular subtense of the retinal image, i.e. the irradiance profile on the retina. This is particularly important when the aperture stop on the imaging system (the eye) reduces significantly the retinal image as compared to the angular sub...