The dependence of the apparent source on exposure position

For a given power entering the eye, the level of retinal thermal hazard depends on the retinal image size over which that power is distributed. Maximum permissible exposure limits are given in terms of the angular subtense of the apparent source, which describese the diameter of the retinal image. Based on a simple beam propagation model for a laser beam being transformed by the eye, it will be discussed that both the location as well as the angular subtense of the apparent source depend on the exposure position of the eye in the beam. For a given position, it is important to consider different accommodation conditions of the eye to determine the location and angular subtense of the apparent source. Only when the eye is fixed in the relaxed condition is the angular subtense of the apparent source equivalent with the far field divergence for any exposure position. For a Gaussian beam, when the eye is located in the far field, the beam waist can be considered as the apparent source, while when the eye is located at or close to the beam waist, the apparent source is located in infinity and the angular subtense of the apparent source becomes equivalent with the far field beam divergence.