Criminal Defense

Forensic Science: Revealing Hidden Indentations Using The Electrostatic Detection Apparatus (ESDA)

January 16, 2024 by Seppi Esfandi in Criminal Defense  Forensic Science  
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ESDA: The Electrostatic Detection Apparatus

In the field of questioned document examination, the Electrostatic Detection Apparatus (ESDA) has emerged as a powerful tool for uncovering hidden indentations or impressions on paper. This non-destructive technique allows forensic experts to detect indentations on pages several layers below the top sheet, even many years after they were made. The ESDA has played a crucial role in overturning convictions and exposing evidence tampering, making it a valuable asset in the pursuit of justice.

How Does ESDA Work?

When writing is applied to a sheet of paper, the indentations or impressions created are transferred to the pages below. ESDA operates on the principle of electrostatic charge and utilizes a specialized process to visualize these indentations. By applying an electrostatic charge to the document and using charge-sensitive toner, the disturbed fibers caused by the indented writing become visible. This technique not only reveals indentations but can also be utilized to develop fingerprints on documents, further enhancing its forensic capabilities.

Unveiling Altered Documents and Tampered Evidence

ESDA has proven to be instrumental in cases where witness statements and evidence have been tampered with. By analyzing the indentations on questioned documents, forensic experts have been able to demonstrate alterations or the signing of blank pages in reverse order to the main notes. These revelations have led to the overturning of numerous convictions and have exposed the extent of evidence tampering in high-profile cases. The reliability and sensitivity of ESDA have played a pivotal role in ensuring justice is served.

“The allegations of tampering with evidence and witness statements led to the unit being disbanded, and over 60 convictions being quashed, many of the appeals relying on ESDA evidence.”

The Science Behind ESDA

The electro-physical mechanism behind the functioning of ESDA is complex. Initially, it was believed that the paper between the grounded platen and the charging film acted as a capacitor, with differing compression of the paper leading to a change in capacitance. However, further research introduced the “charge transport model” proposed by Seward in 1998. According to this model, the detection capability of ESDA is attributed to a surface charge effect created by friction between the paper and the writing instrument. This model has been validated through subsequent testing, which revealed that areas of indentation are less negatively charged than the surrounding areas. The relative difference in potential attracts toner to the indented areas, making them visible.

Practical Application of ESDA

Using the ESDA to visualize indentations is a straightforward process, although certain factors need to be considered to ensure accurate results. The evaluation of the material, adequacy of the paper, and the presence of any wrinkles, creases, or stains are crucial elements to consider before conducting an examination. Additionally, the humidity level plays a significant role, with ESDA performing best at relative humidity levels below 60%. To prepare the document for examination, a humidification chamber can be utilized to achieve the optimal humidity level without over-wetting the document.

Steps in Using ESDA to Visualize Indentations

Evaluation of Material

Assess the adequacy of the material for examination, ensuring it is a single sheet of clean, smooth paper without wrinkles, creases, or stains.

Determine the relative humidity of the environment, as ESDA works best at levels below 60%.

Examination Using Side Lighting

Prioritize the examination of all items using side lighting, as it can detect deep indentations that may not be fully developed by ESDA.


Ensure the document is properly humidified using a humidification chamber, maintaining the ideal humidity level without over-wetting the paper.

Fitness-for-Use (FFU) Test

Conduct a FFU test to verify the functionality of the ESDA by using a control sample known to have indentations.

The FFU test can be prepared by hand or using a specialized device to produce control indentations of varying depths.

Placement on Platen

Place the questioned document flat on the platen surface, with minimal wrinkles or distortions, ensuring it does not extend beyond the platen edges.

Covering with Charging Film

Lay the charging film over the document, taking care to avoid wrinkles or excessive stretching.

The charging film should cover the document completely and make full contact with the platen surface.

Charging of Surface

Use a handheld unit with a high-tension corona wire to electrostatically charge the top surface of the charging film.

Wave the corona wire over the entire surface of the platen to produce an evenly distributed electrostatic charge.

Application of Toner to Develop Indentations

Apply toner to the charging film using methods such as the cascade method, aerosol spray method, brush method, or toner pad method.

These techniques allow the toner particles to be attracted to the indented areas, making them visible.

Assessment of Results

Evaluate the results of the examination, comparing them to the FFU test sample to determine the presence or absence of detectable indentations.

Record and preserve the results, either through adhesive lifts or by photographing or scanning the developed indentations.

Preservation of Results

Preserve the developed indentations using adhesive lifts, fixing the toner between transparent polymer films.

Alternatively, capture the results through photography or scanning.

Additional Applications of ESDA

ESDA can be utilized beyond visualizing indentations. It can also detect fresh fingerprints, fiber disturbance on the paper surface, or footwear impressions on paper.

Manufacturers of ESDA

Several manufacturers specialize in producing ESDA devices. Foster + Freeman Ltd is a prominent manufacturer that offers the ESDA2 and ESDA-Lite models. Projectina manufactures the Docustat DS-220, while the Lightning Powder Company, Inc. produces the Electrostatic Vacuum Box and Vacu-Box™ for Indented Writing. These manufacturers contribute to the advancement of ESDA technology and its application in forensic examinations.


The Electrostatic Detection Apparatus (ESDA) has revolutionized the field of questioned document examination by enabling the visualization of hidden indentations. This non-destructive technique has proven instrumental in uncovering altered documents, exposing evidence tampering, and overturning wrongful convictions. Through the application of electrostatic charge and charge-sensitive toner, ESDA reveals disturbances in paper fibers caused by indented writing. The development and utilization of ESDA devices by manufacturers such as Foster + Freeman Ltd, Projectina, and the Lightning Powder Company, Inc. ensure the continued advancement of this invaluable forensic tool.

ESDA’s ability to reveal hidden indentations and provide insights into document history has made it an indispensable asset in the pursuit of justice. It has been proven to be a crucial asset in solving homicides, among other crimes. As technology continues to evolve, it is likely that ESDA will play an increasingly important role in the examination of questioned documents and the uncovering of crucial evidence.

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