Optical characteristics


Angular Deviation (prismatic deviation, deviation)

Explanation: Angular deviation is the angular change that occurs when a light ray passes through a transparency. The change is usually due to non-parallel surfaces in the transparency. The amount of angular deviation depends on the index of refraction of the material, the angle of incidence, and the shape of the material.It is especially important to characterize angular deviation in aircraft equipped with a head - up display (HUD). When the pilot places the HUD aiming reticle (pipper) on the target, he is aiming his weapon at the location where he visually perceives the target . If the transparency causes angular deviation, the target will actually be displaced from where the pilot sees it, similar to how an object under water appears in a different position from where it actually is.

Inspection: In general, angular deviation cannot be easily detected in the field by optical or visual means. Consistent bias effects in weapons aiming is an indication that uncompensated angular deviation may exist in the transparency. More often the derivative of angular deviation (rate of change of angular deviation) is not iced which manifests itself as distortion (see section on distortion).

Measurement: Measurement of angular deviation is performed with an angular devotion device (ASTM Standard Method F801) or a collimated light source and theodolite. These measurements are done in a laboratory with the windscreen removed from the aircraft. At present there are no methods of measuring angular deviation in the field.

Visual Effect: There is no obvious visual effect of angular deviation; the only effect is an indirect one due to weapons system inaccuracies caused by the angular deviation as discussed above.


Binocular Disparity

Explanation: Binocular disparity exists when the image seen with the left eye is different from the image seen by the right eye. A certain amount of disparity is natural, since the eyes are physically separated. However, excessive binocular disparity may be caused by the transparency or the interaction of the transparency with the HUD, leading to visual problems. Binocular disparity is most often caused by the binocular deviation of the transparency. Binocular deviation is the difference in angular deviation measurements made from the left and right eye positions for a given view angle. Thus it is the angle that the eyes would have to converge or diverge to fixate on an object located at optical infinity. Binocular disparity can also occur when the HUD symbology appears at a different optical distance than the outside target does. This can cause either the HUD symbology or the target to appear widened or double.

Inspection: Binocular disparity is sometimes difficult to notice by visual inspection. It may be detected by alternately closing the left and right eyes and observing a shift in the position of an object.

Measurement: Binocular deviation can be measured by taking angular deviation measurements from the left and right eye position and subtracting the left eye result from the right eye result. This is done for both horizontal and vertical angular deviation directions. The horizontal data provides information on eye convergence or divergence and the vertical data provides information on eye dipvergence (one eye having to rotate upward or downward compared to the other eye in order to fuse the images). It may also be quantified by taking double exposure photographs through the transparency, with one exposure made with the camera in the left eye position and the other from the right eye position (without advancing the film). Separation of the grid lines indicates the presence of binocular deviation. This latter method, however, does not distinguish between lateral displacement effects and angular deviation effects. It is therefore only a good measure of binocular disparity for the specific grid board distance used to obtain the double exposure photographs.

Visual Effect: Binocular disparity may be manifested in several ways: eye strain, headache, fatigue, suppression of the image from one eye by the visual system, or doubling of vision. Sometimes these effects may occur only over a period of extended viewing. Tolerances for binocular disparity vary among individuals, so a certain amount of disparity may cause problems for one individual and not another.



Birefringence (rainbowing)

Explanation: The term birefringence means the material in question has two indices of refraction. Polycarbonate under stress becomes birefringent and thus exhibits two indices of refraction that align with the directions of the stress. These two indices of refract ion cause polarized light to travel at different velocities through the material. The incoming linearly polarized light is converted to elliptically polarized light due to the birefringence. The degree of rotation of the electric field vector of the light further depends on the wavelength (color) of the light since the material also has a certain amount of chromatic dispersion. When the light exits the windscreen, the angle of exit acts like a partial analyzer (polarizer) which results in some wavelengths being attenuated more than others. Thus the exiting light exhibits a color effect depending on the degree of birefringence and the extent of the polarization. These color patterns, or rainbowing, can be relatively strong for clear blue sky days (blue sky can be about 80% polarized). The pattern of these colors on the windscreen remains constant as a result of built in residual stress in the windscreen (during the manufacturing process), but the actual colors making up the pattern will vary depending on the orientation of the windscreen with respect to the polarization vector of the exterior light.

Inspection: Birefringence is visible to the unaided eye when observing the transparency with a polarized light source, such as a clear blue sky. The birefringence pattern can be enhanced by observing it through a second polarizer, such as a pair of polarized sun glasses. (This is why USAF pilots are not allowed to fly with polarized sun glasses.)

Measurement: There is as yet no accepted method of measuring birefringence in terms of its effects on vision.

Visual Effect: Birefringence has been noted as a concern but has not been labeled as a problem. Anecdotal information gathered on F- 111 and B-I windscreens indicates that the primary visual effect is one of annoyance or minor distraction.




Explanation: Crazing is the occurrence of very small "micro cracks" In a transparency or coating. These cracks usually are localized and are oriented in the same direction. In bright light conditions and at certain sun geometries, the cracks will act like many tiny mirrors and reflect light into the pilot's line of sight (see figure 2.5). Crazing may be induced by chemicals, age, or other causes.

Inspection: Visual examination of the transparency under bright light conditions is a good way to observe crazing. However, the appearance of crazing is dependent upon the relative positions of the light source, transparency, and observer, so it may be difficult to observe if the geometry is not right.

Measurement: There is no quantitative method to measure crazing as of this writing. Extent of crazing is left to subjective judgment.

Visual Effect: Crazing can be almost invisible and have essentially no effect on vision until the sun angle is just right and the micro-cracks (acting like little mirrors) reflect the sunlight directly into the pilot's eyes. Under this reflection condition the visual effect is significant loss of contrast in the exterior world scene which can cause severe visual impairment during the time the reflection geometry is satisfied.




Explanation: Delamination is a separation of the layers of a laminated transparency which may be due to residual or induced stress in the transparency. There are several events that may enhance the occurrence of delamination, such as overheating the transparency, thermal cycling, and defective manufacturing.

Inspection: Delamination is detected by looking for bubble areas within the transparency, it often occurs near an edge.

Measurement: There is no specified method to measure delamination, although the width (distance from the edge of the windscreen) of the delamination area is commonly measured using a ruler.

Visual Effect: Delamination is easily noticed but is usually confined to the edges of a transparency (at least in its early stages). This reduces any effect on vision. The area that is delaminated has a lower transmissivity and higher reflectivity due to the extra air- plastic/ glass interface that is created at the delamination. This also enhances the effect of multiple imaging.



Diffraction (streaking, starburst patterns, bowtie effect, arcing)

Explanation: Diffraction is one of the three hasic means by which light rays change their direction of travel (the other two are refraction and reflection). Diffraction is a rather complex subject, but tile effect essentially occurs as a scattering of light from the edges of some obstacle. This scattering can occur from objects too small to see or from easily visible scratches on the surface of a transparency. Diffraction of light from very tiny objects (at the molecular level) is what gives rise to haze. This type of effect is evident in even new materials since it is a characteristic of the material itself (haze or halation is discussed in section 2.9). Diffraction also occurs from inclusions (meshes) and microscratches on the surface of the windscreen. Sometimes these scratches are not in random orientations but are in uniform directions, which give rise to an easily noticeable diffraction pattern. If the scratches are all in one direction or arch, the resulting diffraction pattern will appear as streaks emanating from point sources of light. These patterns are usually only evident at night when viewing point sources of light. This is because in the daytime the daylight scene washes out the pattern effects making them invisible to the naked eye.

Inspection: Diffraction is detected by looking through the transparency at a light source at night or in a dark environment.

Measurement: There is no measurement for diffraction other than a subjective assessment.

Visual Effect: Diffraction patterns are usually only distracting; they are observed primarily at night.



Explanation: Distortion is the rate of change of angular deviation across the transparency. It can be caused by non-parallelisms in the surfaces of a transparency or localized changes in the index of refraction of the transparent material. There are several types of distortion which have specific names within the transparency industry. Some of the more common types are listed here:

1. bullseye - caused by a localized depression or bulge in the transparency, creating a circular lens-like distortion; hence, the name "bullseye."
2. band distortion - distortion occurring in a narrow, elongated region across an area of the transparency.
3. edge distortion - distortion occurring at or near the edge of a transparency. Often the most severe distortion within the transparency will occur along an edge.
4. deletion line distortion - a thermally induced distortion occurring around the heater coating deletion line. A large temperature gradient between the heated and unheated portions of the windscreen may cause localized distortion in some transparencies where the index of refraction varies with temperature.

Inspection: Distortion is readily identified visually by viewing objects through the transparency and noting waviness in lines and changes in the shapes and relative sizes of objects, particularly near the edges of a transparency and in areas where the viewing angle is very acute.

Measurement: Currently three methods for measuring distortion are used within the transparency industry: grid line slope, displacement grade, and lens factor. The most widely used method is grid line slope (ASTM Standard Method F 733 or variations). Grid line slope measurements are made by taking a photograph of a grid board through the windscreen. The maximum slope of a horizontal grid line is the grid line slope value of the transparency.

Visual Effect: The visual effects of distortion depend upon the severity of distort ion. Distortion may be distracting, give false motion cues by changing the perceived relative velocity of out-of- the-cockpit objects, or in some cases cause headache and nausea. Minor distortions, while aesthetically unappealing, have shown no significant degradation on the performance of visual tasks.


Halation (haze, scatter, glare)

Explanation: Halation is the scattering of light by the windscreen into the line of sight of the pilot. It is caused by the diffraction of light by particles within the transparency or by fine scratches and/or dirt on the surface. It is most significant when flying towards the sun and may occlude significant portions of the field of view.

Inspection: Halation is observed by looking through the transparency with a bright light source (or the sun) shining on it. Any veiling glare or scattered light by the transparency that interferes with your view is known as halation, or haze. The amount of haze depends on the intensity and location of the light source and the view angle.

Measurement: Halation may be measured by ASTM Standard Method F 943 in the field and in the laboratory or by D 1003 in the laboratory only. Haze may be quantified as the haze index value (by ASTM F 943) or as percent haze (ASTM D 1003).

Visual Effect: Halation reduces the contrast of objects viewed through the transparency, which makes out-of-the-cockpit objects less visible and decreases the detection range of air-to-air targets.


Multiple Imaging (ghost images)

Explanation: Multiple imaging is observed only at night or in very dark ambient light conditions. It is the appearance of two or more images of a single object or light source. It is caused by light rays reflecting off the inner and outer surfaces of the transparency and back into the pilot's eye. Secondary images may vary in location and intensity with respect to the primary image.

Measurement: The angular displacement of the secondary images from the primary image may be measured following ASTM F 1165. The intensity ratio of the images may also be measured, although a formal procedure does not yet exist.

Visual Effect: In most cases multiple images are simply distracting. In extreme cases, they may give the pilot false motion cues, such as an inaccurate perception of approach velocity or rate of descent during nighttime landing.


Reflections (cockpit reflections)

Explanation: Reflections from transparency surfaces of cockpit objects (flight suit, helmet, etc.) or instrument lights interfere with the aircrew's out-of-the cockpit vision. The reflections are most significant at night or in bright sunlight conditions.

Inspection: Reflections are easily observed visually under a variety of conditions. Measurement: The reflectivity of a transparency may be measured photometrically. A new ASTM standard method is currently being published which details the measurement procedure.

Visual Effect: Reflections on the transparency reduce the contrast of out-of-the-cockpit objects and may even obscure these objects.

 Note: This clasification is based on document AAMRL-TR-89-015