Latent fingerprints can be found on any surface, porous or nonporous. And for ages, they have been a powerful tool to assist forensic investigators in finding the suspect during the criminal investigation process. 

Depending on the type of surface they are present, they can be divided into three categories:

• Plastic fingerprints (prints created on a soft surface like wet paint, wax, or blood due to pressing and are photographed) 
• Patent fingerprints (these are visible to the naked eye and does not require any powder or chemicals for their development)
• Latent fingerprints (these are not visible to the naked eye and several chemicals are required for their development to make them visible). 

Here we will focus on various latent fingerprints development techniques, how the type of surface plays a vital role in selecting the type of chemical, and why it is essential to follow the correct order of chemical usage by forensic investigators to collect the fingerprints.

Developing Latent Fingerprints

Latent fingerprints development is challenging and time-consuming as the prints are fragile, and if the proper procedure is not followed, the outcome will not be satisfactory.

Several factors, including the surface they are present on, temperature, humidity in the air, sunlight, etc., should be considered before you begin to process the prints. 

Out of the various development methods available, there are a few methods that forensic investigators commonly use to discover and process latent evidence.

Nonporous Surfaces

Nonporous surfaces like glass, metals, plastics, etc., are smooth in texture, and the oils and deposits cannot soak into them. Hence, the latent print adheres to the surface only. However, though porous, wood can become nonporous on treating with varnish and paint. 

As the latent prints are present on the nonporous item surface, they are fragile and can be easily destroyed. Therefore the first most crucial step is to carefully preserve the item with the print unless it is handed over to the forensic investigator for processing.

Developed latent fingerprints are one of the most popular forms of evidence presented in a court of law.

Latent Fingerprints Development Techniques on Nonporous Surfaces

Black Colored Powder 

Black powder is an extremely fine powder, and upon adhesion with the latent print, even the minute details are visible. However, a few points should be considered before using it; it should be free from moisture, and the adhesion level needs to be perfect so that it sticks to the moisture present in print effectively. As the moisture can ruin the effectiveness of results obtained after being treated with black powder, you should avoid using it during rain or wet items. 

White/Gray/Silver Colored Powders

These powders are similar to black fingerprint powder and are used in the same way. They help forensic investigators to process latent prints present on darker surfaces to create a contrast between the print and the surface it is present on.   

Fluorescent Colored Powders

Their usage technique is similar to the other colored powders. However, fluorescent powders can create a strong contrast irrespective of the print's surface. One more advantage of using it is that an alternate light source under which the dyes illuminate or glow under specific wavelengths can be used with such powders. It is helpful to capture multiple layered impressions.

Specialized Powders

Latent prints developed should be of high quality to identify suspects or criminals by law enforcement. Hence, specialized powders were introduced to combat forensic investigators' issues with other powders.

Issues With Basic Powders

Black Powder: It is the standard powder used for fingerprint development. However, it is of no use when the surface on which the latent print is present is dark in color.

Light Colored Powders: They help develop prints on darker surfaces, yet these can reverse the ridge detail tone, which might cause visual issues to the examiner.

Fluorescent Powder: Colored fluorescent powders help overcome background issues. However, the bright-colored appearance of the latent print can be of only one color, and this color, if chosen incorrectly, can hinder the identification process.

Hence, C-SiO2 nanopowder offers more significant benefits over standard powders to overcome these problems. Under various wavelengths, it produces different tones and contrast that increases the chance of effective identification. 

Cyanoacrylate (Superglue) Fuming

Sweat, proteins, fatty acids, amino acids and other chemicals are present in latent prints. On being converted to a gaseous state, Superglue or cyanoacrylate gets attracted to these items. A milky white-colored image of the print is obtained on processing, which is much more stable than prints developed with other methods because of glue hardening. 

Dye Stains

It is used in combination with the Superglue fuming method. Once the latent print is processed, the investigator might brush, dip, or spray the liquid dye onto it to make it more visible. It gets soaked into the glue and stains it with its color. Also, the color of the dye used contrasts with the background color on which the print is present. These prints get illuminated using long-wave ultraviolet light and are then photographed.

Porous Surfaces

Porous surfaces like raw wood, paper, cardboard, etc., are ones on which the latent gets absorbed into the surface material. Various chemicals act on the residues like amino acids, proteins, sweat, etc., present in print, and the resultant print produced is in the tone of brown, blue, or purple.

Latent Fingerprints Development Techniques on Porous Surfaces

Iodine

Iodine is present in crystal form, and upon converting it from solid to a gaseous state, the fumes adhere to the oily substances present in print. The outcome is a sharp and clean processed latent print. Also, once the processing stops, the latent print will disappear. 

So, photographs must be taken as soon the prints become visible, and the area on which they are present should also be marked for law purposes.

Ninhydrin

This method is as famous as the cyanoacrylate method of latent print development. It is sprayed on the evidence, or the evidence is dipped in the bath of the solution. The reaction between the amino acids present in the sweat left behind on the absorbent surface, and the Ninhydrin solution results in deep to light purple print production. 

This print can stay for a few days to months. So it is advised to mark the prints and photograph them at the earliest. Also, it can stain the investigator’s skin. So, it is recommended to wear gloves and a protective mask while dealing with this chemical.

Silver Nitrate

It is a liquid-based solution and shows remarkable results in developing prints present on paper, raw wood, and cardboard. It combines with the salts deposited when fingers contact the surface. 

Silver nitrate solution is sprayed onto the evidence containing the latent, or the evidence is dipped in the solution bath. Also, a glass or a porcelain container should be used to prepare the solution and not plastic. 

Once the evidence is treated, it is dabbed with a cloth or paper towel to remove extra solution and is placed in the sunlight. The print will develop like a photographic film. Photograph the print as soon as it gets developed. The evidence should be kept in darkness during photographing, as, in the light, the processing will continue to occur, and the latent print will get overdeveloped.

Developing Latent Fingerprints Present on Human Skin

At a crime scene, the most common visible prints are that of blood that is visible and can be easily photographed. However, sometimes the residue is too light in color that it cannot be easily identified. 

So, chemical methods are used for detail enhancement. The chemicals react with the hemoglobin in the blood, making the print visible. It mostly happens when fingerprints are present on the human skin, which results in poor contrast between the print and the surface; in this case, skin.

Amido Black: It is a dye that produces a dark blue to black print on staining the protein present in the blood. It can react with any protein present and is most commonly used to develop prints on human skin.

Developing Latent Fingerprints from Textured Surfaces

Many times prints are present on surfaces that are rough or not flat. Collecting prints becomes quite tricky, yet it is not impossible.

AccuTrans: It is a liquid casting compound and is applied to the print after being developed using powder. Upon drying, the print can be collected. Latent prints on rough, curved, or textured surfaces can be lifted with it.

Other Latent Fingerprints Development Techniques

Sticky Powder or Wet Powder

It either comes in a powder form to which the solution needs to be mixed or a ready-to-use premix. It is used to develop prints on the sticky side of the duct tape, bandages, masking tape, plastic surgical tape, and similar substances.

It is sprayed or brushed onto the tape and washed off under regular water. Upon rinsing, the solution will get washed from the rest of the tape and stick to the print present showcasing sharp ridge details of the latent print.

Laser Luminescence

In this, the fingerprints illuminate due to fluorescing particles that might be picked up on touching oil, ink, or paint. This method is best suited for processing prints on metals, plastic, wood, painted walls, cloth, and rubber.

Metal Evaporation

Gold is evaporated under a vacuum on the evidence containing print to form a skinny layer over it. After this, the second layer of zinc is evaporated to form a second thin layer over the evidence surface. It does not deposit on print ridges but only covers the background making the print visible.

Bacteria

A nutrient gel containing bacteria is pasted on the painting surface. As the bacteria multiply, the print becomes visible. After that, the gel can be wiped off, and no damage is caused to the painting.

Alternate Light Source (ALS)

It consists of a laser or incandescent bulb, a filter or a combination of filters. These filters break down the light into different wavelengths, and light of one wavelength is absorbed, and filters detect emitted light and block other wavelengths of light.

Investigators use different wavelengths of light with different filters to find latent prints on surfaces like chairs, tables, computers, etc.

The Perfect Order of Collecting Fingerprints

There are several latent fingerprints development techniques present and which one is to be used first is of utmost importance. The forensic investigators must follow the correct sequence of tasks and the correct combination of different methods to get the perfect outcome.

Mostly the investigation starts with visually inspecting the evidence under bright light or alternate light source. After that, determine the steps to be taken depending on the type of surface on which the print is present.

Though a specific step-by-step process is nowhere mentioned yet, the forensic investigator needs to take into consideration several aspects, like the type of suspected print, type of surface, quality of surface, i.e., whether it is clean, sticky, greasy, or dirty; the environmental conditions like whether it is sunny, dry, or humid; the time when the suspect might last touch the object, etc. Once all the factors are noted, the technique and the amount of work required to develop the print can be determined.

Final Words

Latent fingerprint development is one of the many ways that forensic investigators might implement to convict a criminal. It is an age-old technique to find the suspect's involvement in the crime.

To be successful in this field, you must be well-versed in using different latent fingerprints development techniques and other analysis methods. You must also stay up-to-date on the latest innovations in this field.

Fingerprint analysis is quite challenging, and without proper knowledge and skills, you cannot become successful. Hence in-depth, hands-on training is required for this.

So, if you want to become a fingerprint analyst, you can look at industry-oriented postgraduate diploma and certificate fingerprint examination and analysis courses offered by SIFS India, India’s leading forensic science laboratory and training academy.

Also, you can contact our support team anytime to clarify your doubts, if any, about the practical fingerprint analysis program.

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