NOVELTY - The apparatus has a light blocking element (1) defining material activated electronically for blocking transmission of incident light (15). A controller coupled to the light blocking element, a photovoltaic power source and a power circuit detects a specified condition, where electric signal delivered to the light blocking element drives the electronically activated material to block the transmission of incident light upon detection of the specified condition. A conductive light transmitting layer (23) is formed adjacent to the light blocking element within a sealed area.    USE - Light blocking apparatus for a photovoltaic power system for use in building applications and ground mount applications.    ADVANTAGE - The apparatus provides an ability to selectively control voltage of the photovoltaic power sources in series with a string, thus preventing open circuit voltage from exceeding regulatory levels during cold conditions, while allowing the operating voltage to remain within ideal range during hot temperature, and hence maintaining safety of the photovoltaic power system in an economic manner. The apparatus reduces amount of light reaching the photovoltaic power source so as to reduce power without increasing heat dissipation of the photovoltaic power source, while avoiding electrical shock, electrocution and fire. The apparatus provides high level of protection by preventing unsafe levels of voltage and current in the photovoltaic system's wiring and components, thus eliminating a risk of the photovoltaic power source to a person performing maintenance in a cost-effective manner, while obtaining visible confirmation of electrical state of an isolated photovoltaic power source through appearance of the electrically activatable light blocking element.    DETAILED DESCRIPTION - The light blocking element is formed from suspended particle material, electro-chromic material, liquid crystal material or phase dispersed liquid crystal material. The specified condition is selected from a group consisting of a short circuit, a disconnecting device feedback signal, an electrical fault and a user input. An INDEPENDENT CLAIM is also included for a method for operating a light blocking apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic sectional plan view of an assembly containing an electrically activatable light blocking element to be located on light incident plane of a photovoltaic power source.    Light blocking element (1)    Light-transmissive elements (10, 11)    Incident light (15)    Surfaces (21, 22)    Conductive light transmitting layers (23, 24)