Thermal Latent Print Development
Written by Michael E. Stapleton and Kourosh Nikoui   

During the Southern California Association of Fingerprint Officers conference in Burbank, California in October 2009, two speakers on the platform were Adam G. Brown and Daniel Sommerville. Sponsored by Foster + Freeman Ltd., the speakers presented a research project they and their associates conducted at the University of Technology, Sydney’s Centre for Forensic Science, in Sydney, Australia. The project involved the thermal development of latent fingerprints on porous surfaces.


In their review of the literature for their research, Brown, Somerville, et al., found a section in the seminal text, Scott’s Fingerprint Mechanics, that discusses the thermal development of latent prints and cites research done in the 1940s. The technique involved the application of heat by a typical clothes iron to a piece of paper. The heat will char the organic substances in the latent print residue and cause the latent print to become visible. The text explains, “As an intentional means of developing latent prints, heat is not a practical technique. Although heat cannot be considered a practical latent print development technique, a number of arson cases have been solved as a result of latent prints developed by heat.” It was suggested that more predictable results could be obtained with the use of chemicals such as iodine and ninhydrin.

Foster + Freeman Thermal Fingerprint Developer TFD-2 latent prints Michael Stapleton Kourosh Nikoui
Foster + Freeman Thermal Fingerprint Developer TFD-2 latent prints Michael Stapleton Kourosh Nikoui
Foster + Freeman Thermal Fingerprint Developer TFD-2 latent prints Michael Stapleton Kourosh Nikoui

Brown, Sommerville, et al., decided to revisit this discounted thermal development technique and determine whether they could develop more reliable and consistent outcomes. They also shared a personal humorous anecdote during their presentation: As they began to experiment with the application of heat to paper, they were causing smoke and fire alarms to go off in the lab to the consternation of university’s administration. In the beginning, they applied heat to various paper substrates with thermal sources such as hair flat irons, hot-air guns, and furnaces.

The research team discovered that with the application of heat for short durations—before the paper becomes charred—the (still invisible) sebaceous or eccrine latent prints would fluoresce under forensic-light illumination at 505 nm and observed through a 450-nm barrier filter lens. They deduced that the conditions required for the thermal development of fingerprints are rapid heating in air to a temperature between 220° and 300°C. Heating at temperatures below 200°C for longer durations did not yield successful visible print development, while heating above 300°C appeared to be too rapid for latent print development and usually caused paper to become scorched or to ignite.

In conclusion, their research showed that eccrine-rich and sebaceous-rich latent print impressions on paper may be made visible on various paper substrates by the application of heat between 220° and 300°C and when observed under illumination in the 505 nm range of light. They claimed the thermal development of latent prints on paper and other surfaces has great potential as a simple, low-cost, chemical-free method for latent print detection and visualization, particularly in situations where development might not otherwise be attempted for reasons of time and expense.

Foster + Freeman became interested in the work of these researchers and began the commercial research and development of a thermal latent print developer for paper- or cellulose-based substrates. Today, they call their instrument the Thermal Fingerprint Developer, or TFD-2.

Foster + Freeman recently loaned the authors one of their TFD-2 units to experiment with for one week. The kit included the Foster + Freeman Crime-lite 82S Blue (495 nm) and 82S Blue/Green (530 nm). Our research was conducted with the TFD-2 kit at the Fremont (California) Police Department Crime Lab where author Nikoui is employed as the chief forensic specialist. We used the TFD-2 within the confines of a fume hood to vent any smoke created by our experimentation with this instrument.

Foster + Freeman advertises the TFD-2 as an automated, high-throughput device capable of developing fingerprints on large quantities of documents.

Evidence is placed on the motor-driven conveyor tray and passed under an intense heating element. The heating element raises the temperature of the document and causes a chemical reaction between the latent fingerprint and the paper’s surface. This produces a fluorescent byproduct that is visible with the use of a forensic light source in the range of 495 nm (blue) and 530 nm (blue/green). The user has complete control over the progress of development through variation of conveyor speed and heat-source intensity. The benefits are listed as:

Latent prints can be detected in seconds.

  • No chemical process required.
  • The virtually contactless system reduces risk of cross-contamination.
  • High throughput reduces search times.
  • Visible prints feature excellent ridge detail and contrast.
  • Can be used sequentially with chemical treatments, including ninhydrin and DFO.
  • For use at crime scenes or in the laboratory.

We tested various paper substrates to include the following materials:

  • U.S. currency
  • White letter-sized printer paper (8.5 x 11 in.)
  • Binder paper
  • Envelopes (white & manila)
  • Brown cardboard
  • White paper towels
  • Post-it Notes – assorted colors (3 x 3 in.)
  • Thick, white card stock (8.5 x 11 in.)
  • Newspaper

Initially we used sebaceous and amino-acid reference pads to deposit and developed excellent results—with the exception of U.S. currency, brown cardboard, and paper that inherently fluoresced brightly under a forensic light source, such as yellow-colored Post-it Notes. We found the latent prints that exhibited the best contrast between the substrate and the impression were visualized with the Foster + Freeman blue/green 530 nm forensic light and viewed through an orange barrier filter lens.

Go to the Digital Edition to view a video of the TFD-2 in action!

Many of the latent prints we developed contained all three levels of friction ridge detail useful for identification of the donor. Newspapers did not reveal stellar results because the substrate’s background was too fluorescent and did not provide enough contrast to observe the latent print. Paper containing large fibers in the substrate, such as brown cardboard, did not reveal usable latent print impressions. This was because the intense fluorescence of the fibers in this paper substrate lessened the visibility of the latent print.

To get closer to “real world” examples, we had various individuals of a variety of age groups at Fremont Police Department rub their faces to deposit sebaceous oils and eccrine sweat on the palmer surface of their hands and then touch various paper substrates. Ages represented ranged from those in their 20s to nearly 70 years of age.

We estimate that nearly half of the samples yielded latent impressions of various quality and quantity of detail. One officer was running a fever and it was believed that he would most likely leave behind latent prints that would contain eccrine sweat. As he turned in his samples to us, he commented that his latent impressions would contain “a lot of sweat.” In fact, this officer’s latent prints were some of the highest-quality latent prints we developed with the TFD-2. We found they possessed all three levels of friction- ridge detail.

Furthermore, we knew that young adolescent children are generally excellent eccrine and sebaceous secretors. Therefore, we obtained tablet-type lined paper that had been previously touched by adolescent children while they were drawing and making artistic designs. We processed these paper items through the TFD-2 and developed latent prints that possessed all three levels of friction- ridge detail.

We also put some of the TFD-2 tested items through ninhydrin processing. We determined that the TFD-2 had no effect in removing or obliterating latent prints that could be sequentially further processed with chemicals.

In most of the testing, we processed paper types according to the temperatures and the millimeter-per-minute exposure times recommended by Foster + Freeman. We kept the temperature at 100% power, also as recommended by the manufacturer. Foster + Freeman’s recommendations were as follows:

Paper Type Thickness (mm) Optimum Speed (mm/min)
Newspaper 0.03 2,000
Plain Letter-Sized Paper 0.08 1,500
$1 Bill 0.1 1,000
White Envelope 0.24 1,500
Carboard Envelope 1.04 1,000

According to our experimentation, these recommendations seemed to be the optimal settings for these paper types. We also processed some of our paper items for longer run times than recommended, and in some cases we heated them a second time. On some, we developed better contrast in the latent print impression. On others, the over-exposure to heat caused a slight charring of the substrate. In some instances, the over-heated paper would then exhibit a high degree of fluorescence and thereby lessened the contrast or visibility of the developed latent impression in relation to the underlying substrate.

In conclusion, the TFD-2 does visualize latent print impressions containing eccrine and sebaceous secretions on various paper substrates as advertised. It does not require hazardous chemicals and the development occurs within seconds. A forensic light source and appropriate barrier-filter lens will be needed to observe the latent impression and photograph the results.

The end-user should gain experience in the use of the TFD-2 before attempting to process actual evidence. When processing various papers, it is our recommendation that the end-user test latent print development on an identical or nearly identical substrate before subjecting the evidence to thermal processing with the TFD-2.

The TFD-2 will obviously not develop every latent print on every slip of paper. However, it does allow the end-user to use this tool as the first in a series of processing techniques before proceeding further with chemical development to reveal additional latent prints.

If allowed more time with the TFD-2, we would have also attempted to develop latent print impressions deposited weeks and months earlier. Finally, the effects of heating paper to a point where any deposited DNA is destroyed is yet undetermined. This aspect will require further investigation.

In our estimation, this is clearly an example of excellent research and development by Brown, Sommerville, et al., in revisiting an old, disregarded technique of thermal print development. Like Thomas Edison, they started where the last man left off and discovered ways to make this discarded technique work on a consistent basis. Foster + Freeman further developed their ideas and concepts into reality with this new technological tool called the Thermal Fingerprint Developer or TFD-2.

Additional Reading

Brown, A.G., Sommerville, D., Reedy, B.J., Shimmon, R.G., Tahtouh, M., "Revisiting the Thermal Development of Latent Fingerprints on Porous Surfaces: New Aspects and Refinements," Journal of Forensic Sciences. 2009: 54(1), 114-121.

Olsen, R.D., Scott's Fingerprint Mechanics. Springfield, IL: Thomas, 1978.

Song, D.F., Sommerville, D., Brown, A.G., Shimmon, R.G., Reedy, B.J., Tahtouh, M., "Thermal development of latent fingermarks on porous surfaces—further observations and refinements," Forensic Science International. 2001: 204(1-3), 97-110.

About the Authors

Michael E. Stapleton retired after 34 years as a Special Agent with the Federal Bureau of Investigation. For more than three decades he conducted fingerprint identification courses for the FBI. Upon retirement from the Bureau he formed Stapleton & Associates, LLC, a forensics training and consulting company. He also invented the “StapleTube” to simplify latent print photography. He and his associates conduct fingerprint identification courses, latent print development courses, and other advanced forensic training courses throughout the U.S. and abroad.

Kourosh Nikoui is the Chief Forensic Specialist and Manager of the day-to-day operations of the Fremont (California) Police Crime Lab. He supervises and trains the laboratory staff, crime scene investigators, and crime scene specialists. He is certified by the International Association for Identification as a Latent Print Examiner, Senior Crime Scene Analyst, and Forensic Photographer. He has been with the Fremont Police Department for 25 years.

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