5 Must-Have Features in a optical prism

Introduction

One of the most common images for any that has study optics is that of Sir Isaac Newton with a beam of white light going through a glass prism and a rainbow coming out on the other side.  It is one of the most famous optical experiments, not only for its simplicity, but also because it helped Newton set the foundations for his corpuscular theory of light.  

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That small prism that Newton was using is just one of many applications that prisms have in many optical systems.  In the article we will try to describe the four  types of prisms: Dispersive (like the one Newton was using), Reflective, rotation and displacements. As well as some consideration on the manufacturing of prisms.  

Dispersive prisms

A dispersive prism is an optical element used to break up light into its different wavelength components – a phenomenon discovered by Sir Isaac Newton. By doing this, the prism separates light of varying wavelengths, with longer wavelengths (red) deflecting at a lesser angle than the shorter ones (violet). This phenomenon occurs because the prism’s refractive index varies by the wavelengths of the light.

The design of a dispersive prism is critical to its functionality and can significantly impact the quality of the light separation. This article will discuss essential parameters for designing a dispersive prism and provide guidelines for creating an effective and efficient device.

Among the parameters that we need to take into consideration when designing a dispersive prism are:

1. Material Selection: The first consideration in designing a dispersive prism is the choice of material. The material used for the prism should have a high refractive index, as this will determine the extent of the bending of light as it passes through the prism. Common materials used for dispersive prisms include glass and quartz, which have high refractive indices and are transparent.
2. Prism Shape: The shape of the prism is also an important factor in its design. The most common shape used for dispersive prisms is the triangular prism, as this shape allows for the maximum bending of light and provides the highest degree of separation. However, other shapes, such as the rhomboid or the pentagonal prism, may also be used, depending on the application’s specific requirements.
3. Angle of Incidence: The angle of incidence, or the angle at which light enters the prism, also plays a critical role in the performance of the prism. A larger angle of incidence will result in a higher degree of light bending, leading to a more pronounced separation of the light into its component colors. It is important to optimize the angle of incidence for a given application to achieve the best results.
4. Size of the Prism: The size of the prism will also affect its performance. A larger prism will generally result in a higher degree of light bending and a more pronounced separation of the light. However, larger prisms may also be more cumbersome and less practical for some applications. It is important to balance the size of the prism with its performance to achieve the best results.
5. Coating: Finally, it is important to consider the coating of the prism, as this can have a significant impact on its performance. The coating should be optimized for the specific application and should be designed to minimize the amount of light loss and scatter. Anti-reflective coatings are common for dispersive prisms, as they help to reduce the amount of light lost as it passes through the prism.

Dispersive prisms are used in a variety of scientific and technical applications, such as spectroscopy, where they are used to analyze the composition of materials based on their spectral signature. They are also used in optics and telecommunications, where they can be used to control the dispersion of light and correct chromatic aberration in lenses.

The dispersion of a prism is typically measured as the angular separation between two spectral lines of a particular wavelength. The following formula, which assumes a thin-prism, can be used to calculate the dispersion of a prism:

D = (n – 1) * A

where:
D = dispersion of the prism (in degrees)
n = refractive index of the prism material
A = apex angle of the prism (in degrees)

In conclusion, designing a dispersive prism involves considering several critical factors, including the choice of material, the shape of the prism, the angle of incidence, the size of the prism, and the coating. By carefully considering these factors, it is possible to design an effective and efficient dispersive prism that provides high-quality light separation.

Reflective prisms

Reflective prisms can be used in imaging systems.  Due to the total internal reflection, light entering the prism can undergo multiple reflections until they reach an output face.  It is possible to add a reflective surface so the prism behaves as a beam splitter.  Reflective prisms are used to reduce the physical size of an optical system, to redirect the direction of light, and to reform the orientation of an image.

Reflective prisms present lower optical power losses than equivalent systems made with mirrors and are usually easier to align due to the fact that a single element is used instead of several.

Figure 1 shows one of the most common reflective prisms geometries.  In general, if the number of reflective faces is even, we will be creating an upright image, while an even number of reflective surfaces  will create an inverted image.

Different types of prisms and configurations

Right angle prisms are usually used to deviate the direction of light by 90-degrees. It’s possible to use a right angle prism in a Porro configuration when light is incident through the prism’s hypotenuse.  Light will be deflected 180-degrees and flipped.

Dove prisms are right angle prisms with their top part removed.  They can be used to invert images. It is possible to coat the side where light is reflected for optical sensing applications.

Right-angle roof prisms are usually used in binoculars or when a right angle deflection of an image is required. The image is deflected left-to-right not top-to-bottom.

A pentagonal roof prism, deviates the beam 90-degrees without deflection left-to-right or top-to-bottom.

Rhomboid Prisms create an output beam that is displaced from the input beam, but it doesn’t change the direction of the beam, nor does it invert the image.

Porro prisms (either stand-alone or in higher-degrees configurations) are usually used to change the orientation of an image.  They are usually used as erectors in optical instruments like binoculars, telescopes, and microscopes where there are space restrictions.  The degree of a porro system will depend on how many axes the image needs to be altered in

Wedge prisms: Have a shallow angle and can be used together for beam steering in a Risley prim pair

Anamorphic Prisms

An interesting application of prisms is the change of the incident beam dimensions.  This is caused exclusively by the geometry of the prism (e.g. the angle of the incident vs refracted faces), and not the focusing elements or collimating effects like in a lens.  Anamorphic lenses are usually configured in pairs to keep the beam traveling along the optical axis.

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Manufacturing

The manufacturing of prisms usually involves several steps.  Starting with the chosen glass, a series of cuts are done to form a basic prism shape. This stage usually ends in a rough draft of the final product.  The prism will have the shape requested but of poor optical performance.

After that, a series of polishing and smoothing steps of the optical surfaces are needed.  This can take several iterations depending on the optical tolerances requested by the client and their application.  At this stage, antireflection coatings, filters, and metallic layers can be added to achieve the required performance.

A technician is in charge of supervising and evaluating each stage.   Some prism geometries can be bought off-the-shelf but for specific applications or custom made optics, it usually requires a considerable amount of time for testing and manufacturing.

Please let us know in the comments if you have had the need to use custom prisms and what was your application.

When checking a prescription, most opticians have an easy time finding and dotting a lens optical center. The center of the lensometer target is moved until it is in the center of the eyepiece reticle.

Often the prescription that includes prescribed prism ends up passed to someone else with the words "Here, you do this." It's really just as easy to center a lens on the point of prescribed prism as it is to center it at the optical center.

Prism is required when the line of sight must be changed to ensure binocular vision i.e., one fused image from both eyes. Prisms are used to move an image depending on whether the patient has a phoria (tendency of the eye to turn) or tropia (a turned eye). In the illustration on the right, the center of the lensmeter target (lines) is placed on the center of the reticle (concentric circles). This means that the lens is aligned with the on the lensmeter with the optical center along the optical axis of the lensmeter. Then the inking device can conveniently dot the optical center line, the place where the lens has no prism.

When a prescription specifies prism, it specifies the amount of prism, in prism diopters, and the direction of the base (thickest part) of the prism. That means that the lens will be placed with that point of prism at the patient's PD, rather than the point of no prism, the optical center.

Remember, all lenses are prisms i.e., plus lenses are two prisms base to base, minus lenses are prism apex to apex.

The place where the prisms join is the point of no prism i.e., the optical center.Verifying prescribed prism is simple; locate the target center, the point where the mires cross at the point of prescribed prism. The target always moves in the direction of the base and position is dependent on whether its a right or left lens.

When patients look through plus lenses, the base is located at the optical center; in minus lenses, the base is located
at lens edge away from center. The place where prisms join is the point of no prism.

NO prism, lenses are centered at the lens' optical center.

The illustrations below show the location of the base in prescriptions with prescribed prism.

Verifying prescribed prism is simple; locate the target center, the point where the mires cross at the point of prescribed prism. The target always moves in the direction of the base and position is dependent on whether its a right or left lens.For example, in a right lens, 2 prism diopters, base out would look like this.

The illustrations below show the location of the base in a variety of prescriptions with prescribed prism. Remember the location of base in or base out is determined by right and left. In is always on the nasal side and out is always on
the temporal side of center.

PRISM AND CYLINDER Rx's

Locating and verifying prism, in a manual lensmeter, becomes more difficult when the target is formed by a cylinder prescription and even harder to visualize when there is an oblique cylinder axis. At axes near 90 and 180, the vertical and horizontal lines help to align the location of the prism. In cylinder lenses the sphere and cylinder lines are visible separately and the target center and point of prescribed prism must be found by rotating the two powers into focus. In the example below, the Rx is +1.00-2.00 x 90 and 1 prism diopter base down.

First, focus the sphere lines and move the center line to 1 base down, then focus the cylinder lines and align the center cylinder line at the center of the reticle. Rotate the power drum back to see if the sphere lines ahave moved from the original place and adjust them if necessary. Repeat the focusing back and forth so that the position where the two center lines cross are at the 1 prism diopter point on the reticle. Excellence comes from practice.

Again, in non-prism prescriptions the optical center or point of no prism is located at the patient's pupillary distance (PD); in prescriptions with prescribed prism, the point of prescribed prism is located at the PD.

FACTS ABOUT PRISM

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