Optical mirror : a complete guide

&#;Magic mirror on the wall &#; who is the fairest one of all?&#;.

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While this guide won&#;t provide the answer to the Queen&#;s question in Disney&#;s classic Snow White and the Seven Dwarfs, it will give you a glimpse of the optical mirrors available on the market and help you select the best one to suit your needs.

Optical mirror definition

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 millimeter 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.

Types of optical mirrors

Shapes

Under the name &#;mirror&#; are many different types of precision optics that may be sorted as one of below type :

• Flat mirror, it is the simplest form similar to bathroon mirrors with higher precision.
• Spherical mirror, relates to mirrors with a radius of curvature either concave or convex.
• Aspherical mirror, more complex shape mirrors designed to attenuate optical aberration of reflected beam. For example : off-axis parabolic mirrors which are used to collimate a bean à 90° angle.
• Elliptical mirrors, are usually used at a 45° angle where they provide optimum circular clear aperture.
• Concave mirrors, the concave shape increase the reflection that can go more than 99% with an angle of incidence (AOI) of 0°.
• Freeform mirrors
• Prism mirrors, can be used either with outside reflection or inside reflection.

 Substrate material

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.

Mirror coating

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.

Broadband dielectric coatings

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-coated mirrors

Coating metal RefLection Indicative damage threshold Remarks Protected* silver Ravg>97.5% on the visible + NIR, Ravg>96% on MIR+FIR 3J/cm2 Very good reflection in the visible Protected* Aluminium Ravg>90% on the visible + NIR, Ravg>95% on MIR+FIR 0,3J/cm2 A MgF2 layer can be added to improve reflection in the UV (250 to 400nm) Protected* Gold Ravg>96% on the IR 2J/cm2 Without surprise the most expensive of the metal coatings.

*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.

Laser mirrors

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.

Superpolished mirrors

Superpolised mirrors refers to mirrors with very high surface quality (superpolishing) with RMS of less than one Angström and low loss IBS dielectric coatings. They are often used for laser applications.

Deformable mirrors

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.

Hot mirrors and cold mirrors

What are Hot mirrors and cold mirrors ? It is rather intuitive (while sometimes confusing) :

• Hot mirrors are reflecting the IR part of an incident beam and transmit the visible part of it.
• Cold mirrors are transmitting the IR part of an incident beam and reflect the visible part of it.

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.

Where to buy optical mirrors ?

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.

When it comes to bulk eyewear purchasement, ordering custom items and buying stock are the 2 different ways to go. Eyewear brands may prefer customizing their models for branding while many choose stock as it's much faster and easier to operate. If you are buying or planning to buy stock goods of optical frames or sunglasses, a lot of things should be evaluated. Since you'll be buying in bulk quantities, a wrong move could mean a waste of your investment. I have come up with this article to help you with stock eyewear orders.

1. Advantages

The benefits of buying stock eyewear are obvious. They are usually cheaper and can be ordered with a small MOQ. Big teams are not required while the goods can be delivered in a much shorter time. Although the operation is relatively simple, it's necessary to understand the reasons behind those advantages so that you can avoid the mistakes.

1.1 Short Lead Time

Short lead time is a big part of the reason why many buyers choose stock eyewear. Unlike ordering custom items, the product development process as well as the production process are not required when you are buying stocks. It could save you months of time, leaving you only the transportation time to concern.

1.2 Simple Operation

Want more information on Optical Mirrors wholesaler? Feel free to contact us.

Another reason to choose stock over custom eyewear is that buying stocks is much easier. All you need do is select the reliable supplier and pick the right models for your market. You don't need to pay a designer to develop new models and the cost for hiring engineers could also be saved. The whole process could be done by a small team or even an individual.

1.3 Smaller MOQs

Many eyewear buyers are stopped from ordering custom models by big MOQs (Minimum order quantity). So why do manufacturers still set it? Fixed production costs like making the molds are actually the main reason factories are concerned. Because these costs can not be covered with the order quantity below a certain number.

However, that is not the case in stock eyewear orders as the production has already been done. Stock suppliers only consider how to sell them all and it could be done with multiple buyers. But still, most stock eyewear suppliers also have MOQs, only smaller. After all, it's the wholesale business, not retailing. Knowing how the suppliers think, it's not difficult to find out that the more you order, the bigger discount you may get on such purchasement.

1.4 Relatively Lower Prices

Lower prices could be another reason that you're buying stock eyewear. Your supplier should charge reasonable rates so you're able to purchase in sufficient quantities. But to avoid low-price traps, it is necessary for you to understand the potential reasons behind it. I'll list some below for your reference:

• They are produced in a big order so that the unit cost can be lower. This is a safe reason although it won't make the discount very big.
• The supplier just simply wants to sell them soon. And the discount will depend on how soon the supplier wants them to be sold out.
• The supplier think these frames are outdated and not popular anymore. In this case, the supplier also wants to sell them immediately. The chance could be style preferences of the different markets.
• The defective rate of these stocks is high. Be very careful with these goods. Do not try unless you are sure the quality will be accepted.

A reputed wholesaler will offer good deals when buying larger quantities from them. However, a lot of stock suppliers are brokers who don't always sell at cheap prices. It's not strange if you find they are giving higher prices than new custom orders for the same item.

1.5 Less Risks

The development of new eyewear models takes months and it still could fail due to unforeseen problems. This is not the risk you need to bear in stock orders in most cases. You will probably be safe as long as the stocks are not defective products. The challenge here will be checking if this specific model works for your own market. A reputed wholesale supplier will constantly try to ease the sales process to ensure a hassle-free buying process.

2. Disadvantages

Going with stocks may also mean giving up exclusive styles for your brands. And quality controls may be challenging. You can only choose what's available and customer service may not even exist with unqualified suppliers. Knowing the disadvantages helps you understand the limitations. This could improve your business dramatically if you can solve them painlessly.

2.1 Branding is Difficult

The main benefit of branding is you can sell eyewear with added value which was built in exclusive styles with decent quality. However, the biggest downside of ordering stock eyewear is sacrificing the most advanced design trends and high-end qualities. You can not prevent your supplier from selling to others or make sure you are the first one in the market selling them. You have to compete with others for the same models and you may face great difficulties forming unique brand styles.

Experienced eyewear buyers know how to deal with these difficulties with their excellent model picking ability or by working closely with reliable stock suppliers. Also, it doesn't mean you can not do anything about branding the ready goods. Brand names or logos can be printed or even lasered on the products. Packaging is another way to go. But still, it won't be like customized orders of eyewear.

2.2 Qualities are Comprised

It is no secret that most stock suppliers prefer quantity over quality. It's actually decided by the market demands. The customers of the high-end market are just a small part compared to the whole market. On the other hand, stock eyewear is made to be sold a lot. So it has to be economical and the quality could be comprised.

However, it doesn't mean stock eyewear can never be of high quality. The big chance comes with extra quantities produced in high-end custom orders. Or the order has been cancelled somehow after the goods are ready. In these 2 cases, high-end customized eyewear become stocks. Many stock suppliers may use this for advertisement. But sadly, it's very rare to see as those custom orders are usually processed with non-disclosure agreements.

Besides the high-end quality limitation, we may also find it difficult to keep the quality consistency in the long term of stock eyewear business. It's because they are probably from different manufacturers. The best solution could be working with a stable stock supplier with complete categories that can cover all your needs..

2.3 Options are Limited

With stock eyewear orders, you can only choose from what have been offered. And choosing eyewear for your market is not only about the styles, but also the sizes, materials or colors. For example, you may find the style is perfect but the fitting is for Euro while you are selling in Asia. So it helps if your supplier understands your market so that they can recommend right products. And if they don't, make sure they listen to your needs and are willing to cooperate for it.

2.4 Customer Service

To some eyewear brokers, stock orders are just one-time businesses of buying and selling. They don't intend to provide any services or maybe they just don't have the ability or resources to do that. Make sure the company you've chosen is nothing like the ones mentioned above.

Final Thoughts

Eyewear buyers choose custom items or directly purchase ready eyewear based on their own business. The supply chain of the 2 ways could also differ in many aspects as mentioned above. Stock eyewear buyers may aim to develop their brands with unique styles. After all, this is the best solution for value added profit. On the other hand, I see many high-end customers start to do some stock orders under the pandemic due to its flexibility. So it doesn't matter which way you are doing, the key is to know what works for you now and if you are working with the right partner.

Contact us to discuss your requirements of Meniscus Cylindrical lenses wholesaler. Our experienced sales team can help you identify the options that best suit your needs.