Automatic illumination control for multi-objective optical instruments such as microscopes

Claim 1

In an optical instrument having an optical axis, a plurality of objectives selectively positionable along said optical axis, and means, including an illuminator, for producing a preselected light level at a receptor plane positioned along said axis, apparatus for automatically maintaining said preselected light level at said receptor plane, regardless of which of said objectives is positioned along said optical axis, said apparatus comprising: a photocell mounted relative to said optical axis to detect at least a portion of said preselected light level at said receptor plane, said photocell producing a signal proportional to the amount of light detected, said signal changing in response to a change in said preselected light level; monitoring means for receiving and comparing said photocell signal with a preselected reference signal and producing an output comparison signal, said comparison signal changing in response to a change in said photocell signal; trigger means receiving said comparison signal and sending an output signal proportional thereto to said illuminator, said output signal changing in response to a change in said comparison signal to thereby change the output of said illuminator to thereby maintain a preselected light level at said receptor plane; and means responsive to a change from one objective to another, said means being electrically connected to said trigger means to overide and reduce the preselected light level to a further predetermined level of light when any objective is not aligned with said optical axis.