NIST Standards
BEST PRACTICE RECOMMENDATION FOR THE CAPTURE OF MUGSHOTS
Version 2.0
September 23, 1997
The original version of the "Best Practice Recommendation" was initiated
at the Mugshot and Facial Image Workshop which was held in Gaithersburg, MD
on October 23-25, 1995. Developed as a recommendation, the implementation of
the practices and principles described in that document makes the conversion
of existing and ongoing photographic collections more uniform. It contains a
suggested set of procedures and equipment specifications for organizations considering
the purchase of new systems or the upgrade of current systems. The recommendation
is not designed to render current and legacy mugshot collections unacceptable.
Rather, it is intended as a means of establishing or improving interoperability
between mugshot systems.
The information contained in this updated revision of the "Best Practice
Recommendation", Version 2.0, does not alter any of the individual points
that were consensually agreed upon and included in the original version of this
recommendation. It does provide additional details and clarifications for many
of those points and has been supplemented with information regarding depth-of-field
and exposure considerations.
This recommendation reflects a minimum set of common denominators. The provisions
of this recommendation are keyed to the quality aspects associated with the
unaltered captured mugshot image. For new mugshot images being captured, the
specifications contained in this recommendation are equally applicable to realtime
electronic capture of mugshots as well as the electronic conversion of photographic
images. For conversion of legacy files of photographs, most of the provisions
of this recommendation are also still applicable. In the future, it should be
possible to add additional specifications without contradicting any of the current
contents of the recommendation.
POSE
The full-face or frontal pose is the most commonly used pose in photo lineups
and shall always be captured. This pose is in addition to profiles or intermediate
angled poses captured to acquire perspective and other information. For subjects
who normally wear eyeglasses, a frontal mugshot image should be captured of
the subject without glasses. This is required due to the glare from external
illumination. An additional image can optionally be captured of the subject
wearing eyeglasses.
DEPTH OF FIELD
The subject's captured facial image shall always be in focus from the nose to
the ears. Although this may result in the background behind the subject being
out of focus, it is not a problem. For optimum quality of the captured mugshot,
the f-stop of the lens should be set at two f-stops below the maximum aperture
opening when possible.
CENTERING
The facial image being captured (full-face pose) shall be positioned to satisfy
all of the following conditions:
The approximate horizontal mid-points of the mouth and of the bridge of the
nose shall lie on an imaginary vertical straight line positioned at the horizontal
center of the image.
An imaginary horizontal line through the center of the subject's eyes shall
be located at approximately the 55% point of the vertical distance up from the
bottom edge of the captured image.
The width of the subject's head shall occupy approximately 50% of the width
of the captured image. This width shall be the horizontal distance between the
mid-points of two imaginary vertical lines. Each imaginary line shall be drawn
between the upper and lower lobes of each ear and shall be positioned where
the external ear connects to the head.
LIGHTING
Subject illumination shall be accomplished using a minimum of three (3) point
balanced illumination. Appropriate diffusion techniques shall also be employed
and lights positioned to minimize shadows, and to eliminate hot spots on the
facial image. These hot spots usually appear on reflective areas such as cheeks
and foreheads. Proper lighting shall contribute to the uniformity of illumination
of the background described in the exposure requirement.
BACKGROUND
The subject whose image is being captured shall be positioned in front of a
background which is 18% gray with a plain smooth flat surface. A Kodak or other
neutral gray card or densitometer shall be used to verify this 18% gray reflectance
requirement.
EXPOSURE
The exposure shall be keyed to the background. Several areas of the recorded
18% gray background shall be used to verify the proper exposure. The averages
of the 8-bit Red, Green, and Blue (RGB) components within each area shall be
calculated. Each of the RGB means shall fall between 105 and 125 with a standard
deviation of plus or minus 10. Furthermore, for every area examined, the maximum
difference between the means of any two of the RGB components shall not exceed
10.
ASPECT RATIO
The Width:Height aspect ratio of the captured image shall be 1:1.25.
MINIMUM NUMBER OF PIXELS
The minimum number of pixels in an electronic digital image shall be 480 pixels
in the horizontal direction by 600 pixels in the vertical direction. It should
be noted that the image quality of the captured mugshots and facial images will
be improved as the number of pixels in both directions are increased. However,
as images are captured with an increased number of pixels, the 1:1.25 (Width:Height)
aspect ratio will be maintained.
Two considerations must be noted regarding this aspect of the recommendation.
First, the normal orientation of many available cameras is the landscape format
which specifies a greater number of pixels in the horizontal than in the vertical
direction. Unless these cameras capture at least 600 pixels in the vertical
direction, it may be necessary to rotate the camera 90 degrees. Second, the
480x600 capture format exceeds the VGA display format of 640x480. Therefore,
at a minimum, an SVGA specification of 800x600 pixels will be required to display
the facial image. The image will occupy less than the total number of available
horizontal pixels.
COLORSPACE
Captured electronic color facial images are required. Digital images shall be
represented as 24-bit RGB pixels. For every pixel, eight (8) bits will be used
to represent each of the Red, Green, and Blue components. The RGB colorspace
is the basis for other colorspaces including the Y, Cb, Cr and YUV. Additional
color management techniques are available from the International Color Consortium.
Information regarding these techniques can be downloaded from the following
URL: www.color.org
PIXEL ASPECT RATIO
Digital cameras and scanners used to capture facial images shall use square
pixels with a pixel aspect ratio of 1:1.
COMPRESSION ALGORITHM
The algorithm used to compress mugshot and facial images shall conform to the
JPEG Sequential Baseline mode of operation as described in the specification
approved by the ANSI X3L3 Standards committee. The target size for a JPEG compressed
color mugshot image file shall be 25,000 to 45,000 bytes.
FILE FORMAT
The JPEG File Interchange Format (JFIF) shall contain the JPEG compressed image
data. The JFIF file shall then be part of the transaction file for interchange
which conforms to the requirements as contained in ANSI/NIST-CSL 1-1993 and
ANSI/NIST-ITL 1a-1997.
