Glazing for One Way Observation
Optical Ratios; Masking and Observation
All glasses display two images simultaneously: a transmitted image, and a reflected image. This gives a viewer an effect similar to that of a double exposure in a camera. For a one way vision application it is generally required to have the glass appear fully reflective from one side, and to be a transparent viewing window, with little distraction from reflections, from the other side. This is achieved by balancing the glass properties of Reflection (different from each side for a coated product), Transmission, and relative Light Levels on either side of the glass.
When one of the two simultaneously observed images is at least 50 times brighter than the other one, only the bright image is perceived. This is the required property on the “subject” side of a transparent mirror where a “subject” seeing the reflected image of themselves and the room, should not be aware of the very faint image of an “observer”.
When one of the images is about 5 times brighter than the other, then the bright image is easily observed with little distraction from the fainter image. This is the required property on the “observer” side of a transparent mirror where the presence of a faint ghost image of the observer is not an issue.
Note the following formula s apply to any glass type, in any installation. For example; even a single pane of clear glass can be seen to act as an effective transparent mirror when the room side lighting is 600 times brighter than the exterior light level. This can be sometimes observed on a dark night in a residence with normal interior lighting when there are no exterior lights.
Two ratios are defined to illustrate the effectiveness of one way vision glass in hiding the observer (Masking Ratio), and the ability of the observer to see the subject (Observation Ratio).
Definition of terms used in the diagram:
Id = Illumination on the dark side (observer side)
Ib = Illumination on the bright side (subject side)
T = Light transmittance through the transparent mirror (equal in either direction)
Rf = Film side reflectance of the transparent mirror
Rg = Glass side reflectance of the transparent mirror
The one way vision glass must be installed with the dominant reflective surface towards the subject side. Note also that this side must have the higher level of illumination. The optical ratios are defined as follows:
Masking Ratio: The ability of the transparent mirror to mask, or hide, the observer
Masking Ratio = Ib x Rf (reflected image brightness seen by subject) / Id x T (transmitted image brightness seen by subject)
Observation Ratio: The ease with which the subject can be observed or seen
Observation Ratio = Ib x T (transmitted image brightness seen by observer) / Id x Rg (reflected image brightness seen by observer)
Glazing to provide one way vision should be designed with the following characteristics to give optimum performance:
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The formulae give the following Masking and Observation ratios for 8:1 light level.
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These numbers indicate that with an 8 to 1 light ratio, the reflected image of the subject is 53 times brighter than the transmitted image of the observer and hence it will be nearly impossible for the subject to perceive the image of the observer. The observer will see an image of the subject 5 times brighter than their own reflection and will not be distracted by the observer’s own faint reflected image
A successful one way observation application involves the careful use of light levels, direct and indirect illumination, and fabric and wall colour choices.
An 8 to 1 light level is recommended and should be adhered to if possible. Less critical applications may allow lower ratios but the masking and observation properties will be diminished.
SAG can supply one way observation products with improved properties which allow a wall to be completely glazed, from floor to ceiling. With no illumination on the dark side, the light coming through the glass from the bright side will automatically create an 8 to 1 light ratio.
Where an 8 to 1 light ratio cannot be achieved, an additional light of grey glass can be added by either multiple glazing or lamination to obtain a satisfactory Masking Ratio. This will however, reduce the brightness of the observer’s image of the subject.
Type of Lighting
Subject side lighting should be bright and evenly distributed over all walls and furnishings but should not shine directly onto the one way vision glass. Beyond this lighting may be consistent with decor and function of the room. The intent is to brighten the reflected image seen by the subject. Note: do not shine subject side lights directly onto the glass because they will only illuminate the observer and the observation room behind the transparent mirror.
Observer side lighting should be dim with no open light sources (such as un-shaded high intensity desk lamps), or reflections from bright objects such as chrome furniture, visible in a direct line of sight through the transparent mirror. Opaque lamp shades on the observer side are recommended for best results.
Subject side decor should be bright and light in colour and shade to create a bright reflected image.
Observer side decor should be subdued, dark and uniform. Patterns should be minimized in favour of plain materials.
Bright reflecting chrome furnishings should not be used on the observer side.
Note that if the subjects are very close to the one way vision glass less than 600 mm, it may be easier for them to see an observer especially if the observer is also very close to their side of the one way vision glass. On the observer side, it is important to keep people, objects and light sources (such as lamps, flashlights and lit cigarettes) as far back as possible from the one way vision glass surface.
- Airport Security. Baggage Inspection
- Day Care Centre
- Police Identification Line-up
- Retail Store Anti-Theft Monitoring
- Airport Security. Immigration
- Privacy Screening for Ladies
The use of tinted glasses enhance the effectiveness of one way vision glazing for a given lighting condition by increasing the effective lighting ratio (ELR). Wherever a reduction in the apparent lighting level and colour rendition of the subject room is acceptable to the observer tinted glass may prove to be very effective.
Calculate the maximum visible light transmission of a glass that will provide an ELR of 40 when the subject room light level is 120 Ftc and the observation room light level is 15 Ftc.
Wherever subject and/or observer rooms are exposed to differing degrees of natural daylighting which is of course variable, then some means of varying the effective lighting ratios needs to be introduced to maintain the effectiveness of the privacy screening. The control of internal lighting levels to both subject and observer rooms by means of a rheostat (dimmer switch) can be effective.
As with all coated glass products the one way vision glasses should be checked for compatibility with construction and insulating glass sealants.