Ambient Visible Light

T.O. 33B-1-1 2-82 NOTE Contact lenses, sunglasses and glasses with photochromic lens that darken when exposed to sunlight SHALL NOT be worn when performing fluorescent penetrant inspection. Sunglasses reduce the amount of visible light radiating from a fluorescing indication and faint indications may not be seen. Photochromic lens will darken when exposed to black light and will reduce the ability to see small indications. Black light properly filtered is not harmful. The output of a black light bulb is principally at 365 nm and the amount of radiation at shorter wavelengths rapidly falls off. The amount of radiation emitted at or below 320 nm is less than 1 percent. However, this quantity is enough to require a filter since ultraviolet radiation below 320 nm can be hazardous and may cause permanent effects. Germicidal, sun tanning and mineral light bulbs that emit short and medium wavelength ultraviolet SHALL NOT be used for penetrant inspection. While black light does not cause lasting effects, some layers of the eye have a tendency to fluoresce when radiated. This can usually be corrected by positioning the lamp so the radiation is not directed or reflected into the inspector's eye, or the inspector can wear UV goggles. 2.7.4.2 Ambient Visible Light. 2.7.4.2.1 Requirements. Inspection of a part for fluorescent penetrant indications with a black light SHALL always be done under the lowest possible level of ambient light. This increases the contrast between the light emitted from the indication and the background. A low level of visible ambient light is critical for maintaining the sensitivity of the inspection. Ambient light in stationary inspection system booths SHALL NOT exceed 2 lumens per square foot (lm/ft²) of white light (1 lm/ft² equals 1 foot-candle). If a stationary black light booth is not adequate or appropriate, other provisions must be made. 2.7.4.2.2 Measurement. Visible light is measured easily by using photometers or light meters. The light meter responds to electromagnetic energy with wavelengths of approximately 380 to 750 nm. This range extends into the longer wavelength black light and shorter wavelength infrared ranges. Precise measurement is possible with filters that exclude black light and infrared. The unit of measurement is the foot-candle. Another term often used to measure light intensity is the lux, which equals 1 lumen per square meter of surface area. One foot-candle equals approximately 10 lux. 2.7.4.3 White Light. For inspecting parts that have been processed with visible-dye penetrant (Type II), the lighting system in the viewing area shall provide at least 100 foot-candles (1000 lux) of visible white light at the examination surface. Refer to paragraph 2.7.4.2.2 for measuring the intensity of white light. 2.7.5 Inspection Conditions. 2.7.5.1 Dark Adaptation. The human eye becomes many times more sensitive to light under dark conditions. This increased sensitivity gradually occurs when the light conditions change from light to dark. When first entering a dark area from a lighted area, little or nothing can be seen. The pupil of the eye must widen to admit more light, and the mechanism of vision slowly changes. Full sensitivity or dark adaptation requires about 20 minutes. A dark adaptation time of 5 minutes is usually sufficient for penetrant inspection with black light. An inspector entering a darkened area SHALL allow at least 5 minutes for dark adaptation before examining parts. The human eye contains a protective mechanism that further complicates dark adaptation. The pupil of the eye responds very rapidly to bright light. A very short bright light exposure cancels the slowly acquired dark adaptation. Time for dark adaptation must be allowed whenever an inspector enters the darkened station or is exposed to ambient light. A timer capable of measuring this time period should be visibly or audibly available within the darkened area.