&#;Magic mirror, on the wall &#; who is the fairest one of all?&#;.
While this guide won&#;t provide answer to the Queen&#;s request in Disney Classic, Snow White and the seven Dwarfs, you&#;ll get a glimpse of the optical mirrors offer you can find on the market and select the best mirror that may fit your needs.
An optical mirror is a precision optics component made of a optical polished substrate material and a high reflection coating. It can be of various shape and dimensions from less than a millimetre to several meters.
Optical mirrors are found in all the Photonics applications : in astronomy inside every telescope, in lasers by design, in vision equipment, and most of the optical assemblies systems. While their usage is quite simple : reflect the light, their range of application is wide, from UV to IR it can be used to transmit light with a change of direction, to focus light (for example in a Cassegrain telescope) or pumping light in a laser.
Under the name &#;mirror&#; are many different types of precision optics that may be sorted as one of below type :
Mirrors substrate material are usually selected for their good shape resistance to temperatures changes, easiness of coating to bound with it and easiness of polishing to guaranty a good polishing shape. For some high power laser applications, substrate can also be the in LIDT constraint and therefore be selected carefully.
Below table shows most common substrate material for mirror blanks :
Substrate material Remarks Float Very cheap material that comes with acceptable polishing and flatness level for low requirement applications. Fused silica Resistant material with good optical machining properties, very commonly used. N-BK7 / H-K9L Most common optical glass, can be used for high requirement surface aspects. Zerodur Material almost non affected by low temperature, used mainly for space applications. Metals Polished aluminium or brass can be used for some high power laser applications, they are also easier to machined than glass type material and mechanicaly stronger. Plastics Plastic substrates are used in optical applications, but the difficulty to keep a good surface quality and the easy wearing of the parts due to outside environment limit its usage, while this remain the cheapest substrate material.Remark : for some applications where mirrors maybe of important dimensions and the part weight is an important constraint like in space industry for example, a lightening of the part can be done with a structuring of the back of the substrate.
The most important part of an optical mirror is its high reflection coating, it is that coating that will be at the interface of incident beam and reflect it to an orthogonal direction.
With the improvement of vacuum deposition equipment in the last 30 years, layer of dielectric material can be deposit of substrates with a nanometric precision.
The material and the thickness used for each layer enable the trimming of a broadband of wavelengths where the incident beam will be reflected.
It has to be noted that the reflection of such mirror degrades as the AOI increases.
*Metal coatings are protected with SiO2 layers in order to avoid corrosion.
**An additional Chromium layer is usually added between the substrate and the metal to improve adhesion.
The critical specification for optical mirrors to be used in laser applications is their LIDT (Laser Induced Damage Threshold, or in short Damage Threshold).
This specification often measure in J/cm2 defines the energy that can be accepted by a surface area. (for reminder Watts are defining a power so values in Watts should be integrated in the time to be translated to energy).
LIDT should be defined with detail of the type of reference laser : CW or pulse laser, pulse duration, frequency and power.
Usually the coating is the contraint before the substrate, but for high power laser substrate can be wear by laser usage too. The lower the polishing and the higher of impurities in the material will bring to lower LIDT of the substrates.
Deformable mirrors are active optics elements used in adaptative optics. They are typically made of an optical membrane, actuators and an electronic controller.
Their main usage is compensating aberrations, from example of the atmosphère in astronomy, of the eye in ophthalmology or of the cells water in biomicroscopy.
Main suppliers of deformable mirrors are ALPAO and ISP-System for high power applications.
What are Hot mirrors and cold mirrors ? It is rather intuitive (while sometimes confusing) :
In a nutshell : Hot mirrors reflect warmth, cold mirrors reflect cold signal.
They are used to separate visible to IR parts of an optical signal.
Like most optical components optical mirrors are bought from precision optics manufacturers & distributors. It is rather common that the substrate and the coating are made in two different site, therefore buying mirror blank and doing the coating in-house or subcontracting it to other supplier is common practice.
Don&#;t hesitate to contact SINOPTIX for your questions about optical mirrors or to request for a quote.
-Principles of Optical Mirrors
Optical mirrors are essential components in various optical systems and devices. They work based on the principle of specular reflection, where incident light is reflected with minimal scattering or diffraction.
-Types of Optical Mirrors
There are several types of optical mirrors, each designed for specific applications:
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Flat Mirrors&#;&#;have a flat reflective surface and are commonly used for directing or folding light in optical systems.
Spherical Mirrors&#;&#;can be concave or convex. Concave mirrors focus incoming light, while convex mirrors spread it out.
Parabolic Mirrors&#;&#;have a parabolic shape and are used to focus incoming parallel rays to a single point, often in telescopes and satellite dishes.
Elliptical Mirrors&#;&#;have an elliptical shape and can be used to create a point source of light.
Cylindrical Mirrors&#;&#;have a curved surface in one direction and are used for one-dimensional focusing or collimating of light.
-Performance Parameters of Optical Mirrors
To evaluate the performance of optical mirrors, consider the following parameters:
1. Reflectivity: As mentioned earlier, it measures the efficiency of light reflection and is often specified as a percentage.
2. Surface Quality: Surface quality refers to the smoothness and flatness of the mirror's surface, usually described in terms of surface irregularities, scratches, and coatings.
3. Coating: Many mirrors have coatings to enhance their reflectivity, reduce glare, or improve durability. The choice of coating depends on the application.
4. Surface Figure: Surface figure characterizes the mirror's deviation from an ideal shape, which can affect the accuracy of optical systems.
5. Diameter and Size: The size of the mirror is crucial for the intended application, as it determines the field of view and the amount of light that can be collected or directed.
-Applications of Optical Mirrors
Optical mirrors find applications in various fields, including:
1. Telescopes: Spherical and parabolic mirrors are used in telescopes to focus and magnify distant celestial objects.
2. Microscopes: Mirrors are used to direct and manipulate light in microscopes for magnification and observation of tiny structures.
3. Laser Systems: Mirrors are crucial components in laser systems for beam steering, alignment, and cavity enhancement.
4. Imaging Devices: Mirrors are used in cameras and digital imaging systems for reflecting and focusing light onto sensors.
5. Astronomy: Large reflecting mirrors in observatory telescopes capture and direct light from the cosmos for analysis.
6. Industrial Inspection: Mirrors are used for inspecting and examining objects in manufacturing and quality control.
Understanding the principles, types, and performance parameters of optical mirrors is essential for selecting the right mirror for a specific application and ensuring optimal performance in optical systems.
UNI Optics can offer the Laser Mirror with 355nm, 532nm, nm, nm, nm. Others can be available according to customers&#; requirements.
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