Transitions and photograyTransition and Photogray lenses change with the light. Light from the sun contains ultra-violet wavelengths (over 700 nm). It is the ultra-violet that creates a change in the chemistry of the lens. This change causes the lens to darken. Transition refers to plastic light weight lenses. Photogray refers to glass lenses. Both of these photosensitive lenses block over 95% of the ultra-violet rays. When indoors, and out of the effect from sunlight, the lenses lighten up to nearly perfectly clear. When in your car, the lenses are only about 25% dark due to the fact that the car windshield blocks about 50% of the UV. Transitions come in two densities. If you prefer clear lenses when you are indoors, the traditional Transitions lens is best for you. However, if you are sensitive to bright light and prefer a comfortable light tint indoors and you want moderate darkening in the car, then you should use Transitions XTRActive.