Processing Magazine

Cool white light best illuminates stainless-steel process vessels

Delivers more light and less heat; has significant impact on visibility, study shows

February 1, 2014

Aarash Navabi, CPIP, L.J. Star, Inc.

The stainless-steel interiors of process vessels common to food, beverage and pharmaceuticals processing must be subject to regular visual inspection.

Besides verifying vessel integrity, proper cleaning procedures and process-media presence or absence, a vessel’s sight glass is used for observing process-medium characteristics, e.g., if material is foamy, cloudy or wrongly colored. Checks are made to see if chemical reactions have begun or completed, or to evaluate, e.g., the state of fermentation.

Vessel illumination impacts how well these sight-glass examinations are done.

Although light-emitting diodes (LEDs) are becoming common in these environments, many remain unaware that LED color can impact viewing a process accurately.

Process tanks and vessels often are equipped with a luminaire mounted externally and directed through a sight-glass window. Sometimes one port is for light inlet and another is for viewing; sometimes view and lighting are accomplished via a single sight glass. Observations are done with the unaided eye or a camera system is mounted on the sight glass.

A recent study conducted by sight glass supplier L.J. Star compared the relative efficiency of warm-white (relatively red) and cool-white (relatively blue) Lumex LEDs in illuminating stainless-steel tank interiors.

Light and color

In the sense here meant, “color” refers to different frequencies in the electromagnetic frequency spectrum. For example, the wavelength range of the color blue is 400–460 nanometers (nm). The study evaluates what color temperature of white light is best suited for viewing stainless-steel vessel interiors. The two extreme ends of the white-light spectrum, from warm to cool, were tested to demonstrate the associated properties of absorption and reflection.

White light is combined of various colors of visible light. For example, when combined in suitable proportions, red, green, and blue produce a white light. The white light “changes” when there is more of one ingredient color than the others. This quality of light is known as the “color temperature” and is expressed in units of degrees Kelvin (K).

Color temperatures can vary from the most visible violet, on the high end of the visible light spectrum, to red on the low end (with wavelengths of 400–700nm). Although common white light sources may also produce ultraviolet (UV) and infrared (IR) radiation, the focus here is only on the visual aspects of material colors.

As seen by any viewer, the color of most materials represents those portions of the light spectrum that the material doesn’t absorb. In addition, metals absorb specific colors on their surface before reflecting light, based on the metal’s atomic structure and wavelength band gap.

Shiny metal surfaces

Given the characteristics of color and light discussed above, for example, gold metal has a yellow/reddish color viewed under white light. This is so because gold absorbs the blue and green that is mixed with yellow and red to make white. Therefore, the light reflected by the gold contains more yellow and red.

Because green and blue are absorbed — i.e., removed from the light reflected — the light intensity is also reduced. The degree of reduction depends on what was removed.

For example, if a material is visibly red, it absorbs blue (420nm) light. When a 420nm blue light shines on red material, the material absorbs 100% of the blue light, which is converted to heat.

In contrast, silver metal absorbs ultra-violet light, but all other colors are reflected. Therefore, silver offers the greatest reflectivity, which is why it is so often used for mirrors.

With stainless steel, a passive surface layer a few nanometers thick, made up primarily of chromium (III) oxide (Cr2O3), affects the color frequency absorption and reflection, in addition to the effect of its own wavelength band gap.

Chromium oxide absorbs the red and yellow portion of the visible-light spectrum, resulting in the bluish hue associated with stainless steel when viewed under white light.

Industrial light sources

The two major light sources found in industrial settings today are the filament-type halogen lamp and LEDs.

Halogen lamps have been around for years. Their light is “warm” because their tungsten filaments produce a white light that peaks on the warm, or red, side of the frequency spectrum. Although halogen lamps generate a large amount of light, they also consume lots of electrical energy, much of which produces heat rather than light. 

LEDs are increasingly popular because of longer life, lower maintenance costs and minimal interruption. LEDs consume far less power than halogen lamps.

LEDs operate based on electro-luminescence of a semiconductor-metal-doped alloy that is electrical-current dependent. The color of light an LED emits is based on the material’s band gap, attained by doping the semiconductor material with various metals during its fabrication.

Because its light production is based on the metal’s band gap, an LED consumes relatively small amounts of energy and has higher luminous efficacy than other light sources. Luminous efficacy is a measure of efficiency in converting electrical energy to light, as opposed to heat.

Study parameters

LEDs can be fabricated to produce warm white light, cool white light or any color temperature in between. One study goal was to confirm that an LED emitting cool white light produces a higher luminous flux (LF) than an LED emitting warm white light. Luminous flux is defined as lumens per watt.

The study compared a warm-white Lumex model and a cool-white Lumex model.

To compensate for the cool-white LED’s higher luminous flux, current level used to drive the cool-white LED was reduced so that the two light outputs matched. An Extech 600-watt, single-output lab-grade DC power supply drove both LED modules. Both LED lights were placed at a port on the tank, and the luminosity of each was measured at a second port.

Table 1 lists the input voltage and the input current levels employed. A one-foot sanitary connection spool piece measured the initial light output (“Light Output Initial” in Table 1) from one foot away from the light fixture.

Each LED unit was placed on the same 1.5-inch top port of a 20-liter stainless-steel vessel having a three-inch sight glass in its top center. Given that both light and sight glass are on the top of the vessel, light measured through the sight glass comes reflected off the stainless-steel interior surface (“Light Output Reflected” in Table 1).

Each light was measured with an Extech Pocket Foot Candle Light Meter positioned at the sight glass. All measurements were done at the same distance to allow making a direct correlation between lumens and foot-candles (fc).

The results are shown in Table 1. Cool white light is significantly more effective than warm white light for illuminating stainless-steel vessels because more of the light produced is reflected rather than absorbed and converted to heat. The difference in light output from the vessel was nearly 12% or six foot-candles, which has a significant impact on how easily a process operator can examine a vessel’s interior.

Aarash Navabi is a Certified Pharmaceutical Industry Professional (CPIP), a certification accorded by the International Society for Pharmaceutical Engineering (ISPE) that recognizes comprehensive knowledge in the pharmaceutical industry. He works with L.J. Star, Inc. (, a leading supplier of process observation equipment headquartered in Twinsburg, Ohio.