<Li> Submarines: Oftentimes submarines are "rigged for red," meaning that the boat is going to be surfacing or coming to periscope depth at night . During such times illumination within certain compartments is switched to red light to allow the eyes of the lookouts and officers to adjust to the darkness prior to looking outside of the boat . Additionally, compartments on a submarine may be illuminated with red light in order to simulate night conditions for the crew . </Li> <P> Vitamin A is necessary for proper functioning of the human eye . The photopigment rhodopsin found in human rod cells is composed of retinal, a form of vitamin A, bound to an opsin protein . Upon the absorption of light rhodopsin was decomposed into retinal and opsin through bleaching . Retinal could then have one of two fates: it could recombine with opsin to reform rhodopsin or it could be converted into free retinol . The American scientist George Wald was the first to recognize that the visual system expends vitamin A and is dependent upon diet for its replacement . Vitamin A serves many functions in the human body outside of healthy vision . It is vital in maintaining a healthy immune system as well as promoting normal growth and development . The average adult male and female should consume 900 and 700 micrograms of vitamin A per day, respectively . Consumption above 3000 micrograms per day is referred to as vitamin A toxicity and is usually caused by accidental ingestion of supplements . </P> <P> Vitamin A is present in both animal and plant sources as retinoids and carotenoids, respectively . Retinoids can be used immediately by the body upon absorption into the cardiovascular system; however, plant - based carotenoids must be converted to retinol prior to utilization by the body . The highest animal - based sources of vitamin A are liver, dairy products, and fish . Fruits and vegetables containing high amounts of carotenoids are dark green, yellow, orange, and red in colour . </P> <P> Vitamin A-based opsin proteins have been used for sensing light in organisms for most of evolutionary history beginning approximately 3 billion years ago . This feature has been passed from unicellular to multicellular organisms including Homo sapiens . This vitamin was most likely selected by evolution for sensing light because retinal causes a shift in photoreceptor absorbance to the visible light range . This shift in absorbance is especially important for life on Earth because it generally matches the peak irradiance of sunlight on its surface . A second reason why retinal evolved to be vital for human vision is because it undergoes a large conformational change when exposed to light . This conformational change is believed to make it easier for the photoreceptor protein to distinguish between its silent and activated state thus better controlling visual phototransduction . </P>

The dark-adaptation curve indicates that after about 8 minutes in the dark