Imagine looking into a sight glass for a routine tank inspection, and seeing for the first time a lot of scratches and potential points of contamination. What happened?
In a pharmaceutical process, visual inspection is often
used for observing the characteristics of the process
medium and for verifying process steps such as a clean-in-place. Such inspections should show smooth, crevice-free surfaces that cannot harbor bacteria or other contaminants. However, as more operators switch to LED
lights for illuminating the interiors of process vessels,
many operators are alarmed by the sudden appearance
of scratches. In most cases, these are false positives
caused by improper application of LED lighting. This article will explain the science behind this phenomenon and
how to properly apply LED lighting to gain an accurate
view of your processes.
Halogen lighting, the traditional filament-based lighting, has a natural tendency to produce light over a wide
area and spread light in all directions. Under such diffused light, a highly polished stainless steel vessel has
a mirror-like finish. LED light, in contrast, emits light
from a small point and spreads light more directionally.
Undiffused light will reveal tiny imperfections in the
steel surface that are actually inconsequential. Figure 1
shows the difference between diffused and undiffused
Figure 1: Left: Halogen lights emit light from a diffuse source. Right:
LEDs emit light directionally, from a point source.
To illustrate the different effects of diffused and undiffused light, figure 2 shows photos of the same steel surface under diffused and undiffused light. The stainless
steel is polished to a pharmaceutical-industry common
surface finish Ra (roughness) of less than 20 µIn (0.5µ).
Viewed under diffused light, the stainless steel appears
bright and has a mirror like finish (what is viewed is a
perfect reflection). Viewed under undiffused light, the
steel appears dark and has many apparent scratches.
Figure 2: Top: Stainless steel under diffused light. Bottom: Stainless
steel under undiffused light.
To analyze these results, the color photos can be
transformed into grayscale, and then the intensity distri-
The Need to Diffuse Light
for Illuminating Process
Don’t panic! What appear to be scratches on a vessel may be
an illusion caused by improperly applied LED lighting
n By David Star, L.J. Star, and Aarash Navabi, CPIP, Mathew Hildner