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Leck Mitchell, Ph.D., PE

11 Rhine Court

Grand Junction, Colorado, 81507

970-244-8661; Leckmitchell@Bresnan.net

Steve Wendelken, Ph.D.

Technical Support Center

Ground Water and Drinking Water

Mail Stop 140

26 West Martin Luther King Drive

Cincinnati, Ohio 45268

Reference: Justification for adding “Laser” to EPA Method M5331

DETERMINATION OF TURBIDITY BY LED NEPHELOMETRY

September 14, 2015

Dr. Wendelken:

Technology is the evolution and application of science based on research and development for 

newer and better ways of doing business. This is especially necessary when protecting the health 

and quality of life for the 310 million people of the United States. There has been a long history 

of advances in monitoring water. The earliest public use for turbidity measurement began in 

1900 with the Jackson Candle Unit, eventually leading in the 1950s to incandescent light 

sources, and now in 2015 to LEDs and laser light sources providing additional wrinkles to the 

amazing changes in photonics...green lasers at 510 to 540 nm. 

It is essential for suppliers to offer plant operators information and choices which are available 

technically, which may be of benefit, and which have proven over the years to be reliable in a 

large number of applications, including water and waste-water treatment. 

The use of a 525 +/- 15 nm in a LED was selected for method M5331 Revision 1.1 because 

calculations show twice as much scattering of light by small particles as opposed to a 650 nm 

light source and six times the scattering of light by an 850 nm (ISO 7027). The monochromatic 

LEDs proved superior to incandescent and white-light LEDs. As laser technology has evolved, it 

too, has shown superior advantages over both poly-and monochromatic LEDs. 

Lasers provide a stronger and more energetic signal to the scattered-light detector due to its 

stability in wavelength and its tightly focused beam, reducing the amount of stray light the 

detector must contend with. By comparison the LED light beam displays a broader cross 

section, allowing secondary scattering. The more focused beam of the laser allows higher 

intensity with less stray light. Hach suggests that a narrower beam results in higher scattered 

intensity from smaller particles. (Introduction to Laser Nephelometry, p.14) 

There are two factors in suggesting an alternate light source for the M5331 Method. The first 

factor is technical while the second is political. Technically: When lasers in the 510 to 540 nm 
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became available, my major consideration was how to incorporate this technology and still meet 

the specifications for the M5331. Section 6.0 of M5331 specifies a light source of 525 +/- 15 nm. 

Currently, lasers at 520 nm and 532 nm are easily available and within the allowable range. 

Other choices, including the 525 nm laser, may become less expensive in the future and more 

desirable. 

In comparing LED and laser in Method M5331:

• The divergence of the light source in the measurement area is the same for both laser and 

LED.

• The distance traveled is the same in both the laser and the LED. 

• The detector is the same in both the laser and the LED.

• All mechanical features are identical; only the light source is changed.

• Both the LED in M5331 and the proposed laser light source meet all quality control and 

performance characteristics.

• Solid-state, single-chip monochromatic lasers reduce overall costs and could provide 

longer life of the light source than with the 525 nm LED. 

The cross-section of the laser beam has a smaller cross-section than the LED, which will result in 

lower detection of stray background light. This will result in more reliable detection of turbidity 

at very low-levels of concentration, but will not display a detectable difference in conventional

plant operation. Detection at lower levels may facilitate membrane indirect integrity testing. 

The USEPA Membrane Filtration Guidance Manual found all approved turbidity meters 

acceptable for indirect integrity testing of membranes but suggested that a laser turbidity meter

might be superior (Sec 5.2.)

The second factor in adding laser to M5331 is political. Virtually all optical lenses and LEDs are 

now of Chinese origin, which would make availability of these components difficult in the event 

of a disruption in production stability. By contrast, several U.S. companies produce and support 

laser manufacturing, which allows a smooth continuity between suppliers and end users, 

irrespective of global issues.

Notes:

(1) Lasers have an inherent collimated beam output; whereas, a LED requires multiple lenses. 

The M5331design for LED, including a laser, provides multiple lenses for the detector so that the 

detector beam is also collimated in the detection volume limiting stray light going into the 
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detector. Use of lenses with a small detector limits the photodiode noise, which is a function of 

size of the photodiode chip. Lenses produce no noise.

(2) A recent review by Jan Tuner and Lars Hode covering scientific and medical journals 

indicated LEDs may be more destructive than lasers when used on organic structures, which 

might have some effect on NTU and should be further investigated. (Laser Therapy, Clinical 

Practice and Scientific Background, Prima Books, 2000, pp. 335-338)

(3) Mitchell USPTO Patent # 7663751: NEPHELOMETER INSTRUMENT FOR 

MEASURING TURBIDITY OF WATER: February 16, 2010, states that wavelengths at 525 +/-

15 nm provides for use of solid-state LED, OLED, and laser light sources. The addition of 

“laser” in the M5331 Method would already be covered under the Mitchell patent.

Respectfully submitted,

Leck Mitchell