There are numerous types of scales to be selected from. Some are appropriate and others are unsuitable. When selecting a scale, there are several questions to consider; what type of lighting is to be used, precision of measurement required, rigidity needed in scale, size of item and thickness of object being photographed. With the advent of computerized databases, search systems and electronic measuring, the need for more precision in the photographs and scales used is becoming apparent. A discrepancy in the distance between two points may bias a data base search or skew data. One should always strive to achieve as much accuracy in the photographs as the system requires.
When selecting a scale, always choose the scale that will provide at least as precise a measurement as the system that will be measuring the photographic information. If the system has a precision to one millimeter, make sure the scale used is accurate and precise to at least one millimeter across the entire scale. An argument that is sometimes presented by attorneys is that the scale used, in the photograph, is not a “certified” scale by the International Standards Organization. One way to circumvent this position is by using the same scale that is in the photograph to measure with when printing those photographs to a particular magnification. It is recommended that your scales be individually identified and marked so that there is no confusion as to which scale was used in the photograph.
Scales that have been found to be generally unsuitable for accuracy or reliable use include: retractable tapes, cloth tapes, wooden and plastic school type rulers and folding carpenter rules. Also not acceptable are those “emergency” scales: pens, coins, badge cases, etc.
Consider the item being photographed and the plane upon which the information to be measured is on. The scale should be placed upon the same plane as the information that is to be measured. If the plane runs on an angle, that angle must be matched to the scale. The scale does not necessarily need to be physically placed upon the surface of the item, but may be placed along side the object or photographically suspended in space near the object; as long as it is on the same plane as the information. If there is some thickness to the scale, make sure that the surface that has the ruled markings is on the plane of the information, not the bottom of the scale.
If the scale is to be placed on the same surface as the information being recorded, make sure that the scale does not obscure any information present. If there is a possibility of trace or bio evidence being present on the surface, it is advisable that self-adhering scales not be used and another type of rigid scale be placed along side the surface being photographed.
Machinist Scales – this type of scale is very accurate and comes in a variety of measuring units. Scales manufactured by Starett and Lufkin have a special surface called “chrome-clad” and “satin-clad” that retain a neutral gray tonality when photographed. It is almost impossible to photographically “wash out” the detail in the scale and they will not photograph black as the stainless steel scale. They are also very easy to maintain and keep clean. These scales are made of spring steel and can either be purchased in a flexible or rigid state in varying thickness. They may also be purchased with different measuring units on either side of the scale, such as 64ths on one side and 100ths on the other or millimeters and half millimeters. Just about any combination of units may be found and lengths from six inches to three feet. Scale price is in the range of approximately $2.00 to $3.00 an inch. Although a little pricey, these scales are some of the best for accuracy and precision.
Plastic Forensic Scales – usually available through medical or forensic catalogs. This type of scale may be useful, depending on the level of precision required. They come in a variety of colors, measuring units, thickness and have some useful indicators incorporated into the scale. Color choice may be important depending on lighting selected, subject tonality and agency photograph is being made for. These scales can be obtained with black fields and white units, white field with black units, or gray field with black units to match subject contrast and lighting technique. Sometimes these scales will have circles with cross hairs to signal any perspective distortion in the image. Alternating contrasting bars of specific lengths are sometimes also incorporated into these scales in case photographic exposure makes reading the measuring units difficult. An 18% gray patch might also be present to provide a densitometer/analyzer reference for color printing and sensitometry.
Self – Adhesive Scales – scales come in rolls of specific lengths, inches or millimeters, and are made to be “peeled-off” and stuck to the surface of an item. It is suggested that the accuracy of the scale be ascertained prior to use. Some can be fairly accurate while others have been found to be off by as much as one and one-half millimeter over a length of six centimeters. trace and bio evidence concerns should be addressed prior to use of the scale. These scales are particularly useful with vertical surfaces and fingerprints. Scale lengths may be obtained from one inch (or millimeters) to continuous that is torn-off at the desired length. There is usually room to write case information onto the scale. There are several suppliers that will preprint agency logo onto this type of scale.
Specialty Scales – this type of scale has been designed for specific applications, but may also be used for any type of scaling need …
Designed by the American Board of Forensic Odontology for use in bite mark photography and is considered the standard measuring device for this application. This is an L shaped scale with measuring units on the inside of the L. Included are cross-hair circles, contrasting measuring bars and 18% gray reference patch. The width of the L’s is one inch to accommodate measuring if exposure errors occur.
L Shaped Scales
There are several L shaped scales available, that are not the ABFO #2, designed primarily for footwear impressions. Smaller versions have been produced for evidence photography and may be used for fingerprints, but have limitations depending on physical size and shape of item or surface being photographed. These scales usually have the perspective circles, contrasting bars and reference patch incorporated into the scale.
Transparent and Translucent
When using trans-illumination, dark field or light field lighting, this type of scale is needed. Light needs to pass through the scale in order to observe the measuring units. An opaque scale will not work unless the scale is lighted independently. Some companies produce a scale that has the measuring units printed on the bottom surface of the scale, placing them closer to the plane of information. Generally, the translucent scales seem to provide a better density than the clear scales, allowing less flare to occur.
Designed primarily for alternative light sources (ALS) and is usually the stick-on type. Scales will fluoresce when illuminated with an ALS, have alternating contrasting blocks and perspective circles.
All of these scales, with the exception of the machinist scales, may usually be purchased from forensic suppliers. The machinist scale should be obtainable from any supplier of precision tools and measuring devices. If you encounter difficulty finding a source, call your local Crime Laboratory’s Forensic Imaging Unit for advice.
Ronald L. Groffy
Forensic Imaging Specialist