What is the Transmittance of Achromatic Lens?

 At present, there are many manufacturers producing achromatic lenses in China. Although the products of each company are called achromatic lenses, the raw materials and technology are quite different, which results in the different transmittance of achromatic lenses in the market. So how much light transmittance can achromatic glue lens achieve?

In fact, achromatic lens refers to a kind of photoelectric glass which combines liquid crystal dimming film and glass clip glue and has the effect of "electrifying transparency, power off and grinding opacity".

When purchasing achromatic plywood lens, one of the most concerned parameters of customers is the transmittance. The higher the transmittance, the better the product. The transmittance of the achromatic lens is mainly affected by the following three aspects: glass original film, film clip and dimming film.

First of all, glass. Glass is mainly divided into white glass and super white glass. Plain white glass contains iron oxide, manganese oxide and other metal oxides. It looks green. Generally, its transmittance rate is 82%~84%. Ultra-white glass contains very low metal oxides. It looks translucent and elegant. Generally, its transmittance is 89%~91%. But at present, the transparency of ultra-white glass produced by the most advanced equipment is only 93%.

Then let's talk about film. There are two kinds of film for the clamping of achromatic glue lens: EVA and PVB. The transmittance of better transparent film can reach more than 90%. The transmittance of films with different qualities and colors is quite different, which is not to be discussed here.

The transmittance of dimming film is the most important factor affecting the transmittance of achromatic lens. There are domestic and imported dimming films. The transmittance of domestic films is generally between 73% and 83%. The transmittance of imported films is generally higher than that of domestic films. The transmittance can reach 85% or higher, but the price is also rising.

 

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The Benefit of Aspheric Lens-Spherical Aberration Correction

 The most significant advantage of aspheric lens is that it can correct spherical aberration. Spherical aberration is caused by focusing or aligning light with a spherical surface. Therefore, in other words, all spherical surfaces, whether or not there are any measurement errors or manufacturing errors, will have spherical aberrations. Therefore, they all need a non-spherical or aspherical surface to correct it. By adjusting the conical constant and non-spherical coefficient, any non-spherical lens can be optimized to minimize aberration. Spherical lens is a lens with significant spherical aberration, not a spherical lens with almost no spherical aberration. The spherical aberration in a spherical lens will focus the incident light at many different fixed points, resulting in blurred images. In aspheric lenses, all the different light rays focus on the same fixed point, thus producing images with less blur and better quality.

In order to better understand the difference of focusing performance between aspherical lens and spherical lens, the point or blur of light produced by two equal lenses (f/1 lens) with a diameter of 25 mm and a focal length of 25 mm, parallel and monochrome light (wavelength 587.6 nm) on the comparative axis (object angle of 0 degree) and off-axis (object angle of 0.5 degree and 1.0 degree) were observed. It is known that the spot size of aspheric lens is several orders of magnitude smaller than that of spherical lens.

 

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Additional Benefits of Aspheric Lenses

 Although there are many different techniques for correcting aberrations caused by spherical surfaces on the market, these other techniques have far less imaging performance and flexibility than aspheric lenses can provide. Another widely used technique involves adding lenses by "shrinking" them. Although this can improve the image quality, it will also reduce the light flux in the system. Therefore, there is a trade-off between the two. On the other hand, when using aspheric lens, its additional aberration correction supports users to achieve high luminous flux (low f/ and high numerical aperture) system design while maintaining good image quality. Image degradation caused by higher luminous flux design is sustainable because a slightly reduced image quality still provides better performance than a spherical system.

System advantage

Aspheric lenses allow optical element designers to use fewer optical elements than traditional spherical elements to correct aberrations, because the former provides more aberration correction for them than the latter uses multiple surfaces to provide aberration correction. For example, a zoom lens with ten or more lens elements can replace five or six spherical lenses with one or two aspherical lenses, which can achieve the same or higher optical effects, reduce production costs and reduce the size of the system. Optical systems with more optical elements may have a negative impact on optical and mechanical parameters, resulting in more expensive mechanical tolerances, additional calibration steps, and more requirements for antireflective coatings.

All of these results will ultimately reduce the overall practicability of the system, because users will have to constantly add support components to it. Therefore, adding aspheric lens to the system (although the price of aspheric lens is more expensive than the same single lens and double lens), will actually reduce the cost of your overall system design.

 

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What Parameters should be Paid Attention to in Aspheric Lens Selection?

 1. Designed wavelength: the wavelength that makes aspheric lens achieve the best effect. Aspheric lenses can also be used at other wavelengths.

2. Numerical aperture (NA): Measure the maximum angle of light that a lens can accept. NA = sin (Φ)

3. Pass-through aperture: According to the design, the maximum diameter of light energy passing through the surface. It is smaller than the actual spherical diameter, i. e. physical aperture (PA).

4. Effective Focal Length (EFL): The distance between the main plane and the focus. The normal tolerance is (±1%).

5. Amplification factor: the ratio of image size to object size. Calibration lenses are not limited.

6. RMS WFE: Measurement value of chromatic aberration (error) in lens. Diffraction limitation means that due to the diffraction, the lens conforms to the theoretical limitation (the highest level of operation that can be achieved in accordance with the physical laws).

7. Outer diameter: the diameter of the outer cylinder of the lens, the standard tolerance is (±0.015 mm).

8. Working distance (WD): Also known as Back Focal Length (BFL), which represents the distance from the back of the lens to the focus.

9. Central thickness: Thickness in the Axis of the Lens.

Aspheric lens is superior to spherical lens.

1. A non-spherical lens can replace the combination of multiple spherical lenses, thus reducing the number of parts.
2. The fewer lenses in the system means fewer assemblies and adjustments, which saves labor time.
3. The smaller number of lenses in the system means that light will illuminate less lens surface, reduce backlighting and improve the optical transmission in the whole system.
Less lenses mean smaller and lighter systems.

 

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Different Advantages of Each Aspheric Lens Type

 Since the lens performance required for all applications is different, it is very important to choose an appropriate aspheric lens. Key factors to consider include your project schedule, overall performance requirements, budget constraints, and projected quantities.

Spot lenses are available immediately, so many applications may be satisfied with the use of off-the-shelf aspherical lenses. However, these standard aspheric lenses can be quickly and easily modified by antireflective film or can be reduced in size to meet the requirements of standard products. If spot products are insufficient to meet the demand, customized aspheric lenses can be used for prototyping, pre-fabrication or mass manufacturing applications.

Here are the advantages of different types of aspherical lenses

1. Precision glass forming aspheric lens

It is very suitable for high-volume production, because it can produce a large number of lenses quickly, and the tool maintenance cost is low.

2. Precision polished aspheric lens

It is very suitable for small batch production, because the delivery time is short, only a small number of special tools and settings are needed.

3. Hybrid aspheric lens

It is very suitable for multispectral applications because spherical aberration and achromatic aberration correction can be provided simultaneously.

4. Plastic moulded aspheric lens

It is very suitable for high-volume production and is a low-cost, light-weight alternative product for aspheric lens.

 

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Advantages and Disadvantages of Aspherical Lenses

 The surface radian of aspherical lens is different from that of ordinary spherical lens. In order to pursue the thinness of the lens, it needs to change the surface of the lens. Aspheric lens should be designed as flat as possible. However, the optical properties of flattened lenses, even if they are designed with aspheric surfaces, will decline rapidly. At the same time, the lens is lighter, thinner and flatter, and it still maintains excellent impact resistance, so that the wearer can use it safely.

Advantages of aspherical lenses:

1. Optical advantages: reduce the aberration of the lens and make the vision clearer.
2. Clear images can also be obtained at high luminosity: Although the spherical point focus lens is designed by the best base arc, the luminosity beyond + 7.00D-22.00D is not within the Cherning ellipse, and the aberration can not be eliminated. Only aspheric design can achieve better quality.
3. It can make the lens flatter, thinner and more beautiful. The higher the lenticity of spherical lens, the worse the appearance. Aspheric lens can be designed with flat base arc, which not only makes the appearance beautiful, but also reduces the peripheral magnification. Let others see the wearer's eyes, will not change a lot in size.

Defects of aspherical lenses:

1. Aspheric lens has a relatively small light area. When the eyeball rotates around, it will blur a little when looking at the outside through the lens edge, that is, the visual range of the line of sight becomes smaller.

2. Human's eyeball is spherical. The eyeball rotates to the edge. Through aspheric lens, the object near the eye appears protruding.

 

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How to Design and Manufacture Aspherical Lens?

 Aspheric fabrication is different from traditional spherical fabrication. In traditional spherical manufacturing, the surface is defined by a single radius of curvature. Curvature can be ground and polished using large tools. This polishing tool smoothes the relative curvature of the finished product surface into the desired shape by oscillating and polishing the curvature center of the medium such as slurry.

1. Selection of aspherical lens materials

CNC and MRF optical glass and crystal materials; in addition, plastics can be chosen. Diamond turning is suitable for crystals, plastics and metals. Diamond turning works well for infrared optical materials and is inherently insensitive to higher surface roughness. Precision glass forming is limited, transition temperature < 500 degree C and thermal expansion coefficient of glasses are low enough, in a very limited choice of glasses. Choices have increased dramatically over the past decade, but still account for only a small fraction of all available optical glasses.

The glass softening temperature is lowered to room temperature in the forming process. In a short time, the glass is annealed and its index is lowered. This decline means that finished lenses will not have the same index as the glass catalogue specification. When designing precision glass moulding, users need to consider the low index in their models.

2. Design of curvature radius of aspherical lens

Compared with spherical lenses, aspherical lenses do not have a single radius of curvature. The edge change of the central part of the radius of local curvature. This leads to the possibility that the local radius of curvature can change the different points of its symbols from convex to concave on the surface, as shown in Figure 3. The place where the point occurs is called a turning point, which may lead to mold and measurement problems. In Zemax software, the cross section of the surface curvature can be used to check the local curvature radius of the surface.

 

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