A Study on the Colored Glass Lens
Doo Hee Han
Industrial Technology Convergence Research Institute, Chungwoon University, Incheon, Korea
To cite this article:
Doo Hee Han. A Study on the Colored Glass Lens. Optics. Vol. 4, No. 6, 2015, pp. 48-54. doi: 10.11648/j.optics.20150406.13
Abstract: Several coloring technologies of ophthalmic lenses were discussed. Dipping, casting, hard coating mixture mixing and multi layer coating method were discussed. Samples of few tens were made and tested. We tried to get the digitalized values of each color dressed lenses. This trial will give a helpful way to make the standard colors of lenses.
Keywords: Lens, Spectacles, Coating, Anti-Reflection, Emi
Eyes are one of the most important parts of our body. Largely dividing functions of the eyes into two, the first would be realizing the colors and shapes, and the next would be determining the distance. Along with the development of civilization, glasses became a good tool for students preparing for entrance exams or for elderly seniors. Unlike the form of a general lens, glasses use converging meniscus or diverging meniscus form. This is for natural direction of light entering to the eyes depending on the movement of the eye balls. Glass lenses shall have good light transmittance with constant refractive index for every surface, and shall not have any bubbles or scratches on the lens surfaces. And there must not be any distortion on the lens surfaces. With the optical properties of glass lenses, glasses became a means of presenting individuality to the young. When considering the design elements in glasses, they would be the glass frame and lenses. The expression of individuality by glass lenses can be largely distinguished by the difference in the appearance of heaviness depending on the lens material, and in the atmospheric change depending on the lens color. Lenses are in a trending to become thinner and lighter as the material is changing from the conventional heavy glass to high refractive plastic. Keeping in pace with the trend, glasses became functioning as ornaments such as rings and necklaces with the use of multicolored lenses. Driven by the background and with a belief that the company who dominates the color can survive in the era of infinite competition, a method to systematize and standardize the lens color according to the process by producing lens color specimens with a small business eyewear manufacturer is researched and reviewed in this study.
1.1. Recognition of Colors
Although eyes can be considered as accurate optic devices composed of lens, iris, retina, etc., unlike general lenses, they retain an excellent performance that do not feel chromatic aberration even though they consist of just one lens. Colors are recognized when the signals of light and colors delivered from the eyes arrive to the brain. The rod cells and cone cells that absorb light convert light with an electrical function delivering it to the bipolar cells.
These stimulations are systematized into color codes through an instant action while being delivered to the visual cortex positioned at the back of the brain . Blue light stimulates blue cone cells and the output generated here stimulates blue and yellow ganglionic cells. The yellow light which stimulate red and green cone cells stimulate them. Thus, the large amount of stimulations from these ganglionic cells compose codes for the yellow light, and the small amount of impacts compose codes for the blue light. The red light that goes through the red cone cells stimulates red-green ganglionic cells. On the other hand, the green light that goes through the green cone cells inhibits them. The minor irregular impacts create codes for green and a large amount of impacts create codes for red .
1.2. Color Function
A color function is a wave function which is also the tristimulus value of the same energy spectrum.
1.2.1. XYZ Tristimulus Value
Tristimulus Value is determined by the color function defined by the CIE in 1931 and is considered as the 2. XYZ tristimulus value. XYZ values are appropriate for a field of vision of 4. or less, and are defined by the reflector expressed as the following function.
Here, S(λ) is an intensity distribution relative to the illuminant are the color functions of CIE 2° standard observer (1931), and R(λ) is the spectral reflection of a specimen. The color coordinates xyz are obtained from XYZ tristimulus values using the following formula.
1.2.2. Hunter Lab Color Space
The Lab colorimetric was developed by R. H. Hunter in 1948 to enable direct reading from the photoelectric calorimeter. These values of colorimetric are obtained from the following formula.
Above formula can be expressed for 2° standard observer and standard light source C as follows.
In the Hunter Lab colorimetric, the color difference is expressed as
and here , , are the difference between the color of the specimen and target .
1.2.3. A Comparison of Color Detection of the Eyes and Colorimeter
1. Eyes: When a light is reflected from an object, the light enters into the three types of cone cells which reads red, green, and blue respectively allowing the brain to detect a combined color.
2. Colorimeter: The light from the standard illuminant reflects from an object, and the three sensors express X, Y, Z or the Lab values displaying in numbers. . I have emphasized in the past that interdisciplinary research is preferable for the study of colors but needs the understanding of the spectroscopy including modern physics. In the past, I have found many errors in explanations done within a single perspective while studying a textbook of the study of colors. .
1.2.4. A Comparison of Color Detection of a Digital Camera and Colorimeter
Meanwhile, a generalized colorimeter is made handy for measuring opaque solids or transparent liquid. Thus, it is not useful to measure big and transparent solid objects like glass lens. However, only if the problem of light source is solved, these can be used in colorimetric devices since the CCD method of scanners or digital cameras are similar to the method of the colorimeters. If the light source is an accurate white light, it will position on an identical radial axial in the ab space with the L value difference according to the luminosity. Although a digital camera was used based on this conception, I disclose that color correction was not done here and the image from the device was expressed as values read by the computer. The color recognition was compared between the colorimeter and digital camera using three colored paper sheets. The sunlight and a tungsten lamp were used for the light source.
|Color gauge||Digital camera|
2. Glass Lens Coloring Technique
2.1. Wetting Method
This method is mainly applied to plastic lens without surface enhancement coating (hard coating, hereinafter). It is not efficient for hard coating lenses since the pigment soaking speed is slow. A lens is dipped for an adequate time into a mixture of a pigment and pure water set to a temperature slightly lower than the boiling degree. When the temperature is high, the pigmentation speeds up but color control gets more difficult instead. It is preferable to create color by dipping into each different pigments rather than mixing them. This is because when mixing pigments, management of color becomes difficult and the different components may cause chemical reactions. When mixing pigments, each dye of different colors are made and the lens is dipped in order respectively, and here, before dipping into another pigment, the lens should be washed clean with around 96℃ of pure water. After the desired color is expressed, the color shall be verified and controlled in a room temperature.
2.1.1. Two-Tone Color
This is a method to express two colors of a same type or a similar type on a single lens, and the ⅔ portion of a lens is the best position for appearance. Two types of colors are expressed mainly using the density of a color, and the boundary shall not be clearly appeared. This means that the lens shall not be in one same position for a long time, and to do this, an oscillation is required in a regular interval.
2.1.2. Incremental Gradation Color
Currently, creating 7 types of tones is mechanically possible for the gradation color, and more types are even possible with computer control. However, the more divisions there are, the more difficult the control becomes and an accurate processing is required as the position adjustment is difficult when processing the lens.
2.2. Color Change by Wetting Method
2.2.1. Color Change Depending on Time
The lenses used prior to processing for making specimens were CR-39, and for the pigments, liquid pigments from BPI were used. Since glasses are transparent objects and solid, it is difficult to use with the colorimeter, thus, digital camera was used. After an image is obtained with the camera in the white light, the image was inserted into the computer to read the color information in the image analysis and editing program. Photoshop 5.0 was used for the color analysis, and reading color information such as RGB, Lab, CMYK etc. were available in this program. However, since the light source used here was not standardized for the luminosity of the lighting etc, a lens for comparison was shot together to lessen the effect of condition changes. Though the light source settings are different between the colorimeter and digital camera, their CCD methods are the same. Thus when using the sunlight or the standard illuminant, colors are read to a close value. The Lab value in the Photoshop program of image shot by the digital camera was used as the direct comparison within the lens which is a transparent object is difficult. To observe the time color difference according to time and mixture by the pigment types, specimens were made using plano lenses.
1. Pigment BPI#32500 is used. The pigmentation time is relatively short and due to the nature of the color, the difference in luminosity is small.
2. Pigment BPI#32100 is used, and is the shield brown type which has fast color absorptivity. Has severe luminosity change.
3. Pigment BPI#22600 is used, and this is an ultramarine type color. The luminosity change is fast.
4. Pigment BPI#49900 is used. The pigmentation time is fast so is the luminosity change.
2.2.2. Two Tone Colored Lens
1. Undiluted two tone colored lens.
This is an example of a two tone colored lens using a single type of a pigment.
2.3. Method of Casting
This is a method to combine pigment with the monomer prior to molding the lens. The limitation is that it is difficult when water is used to maintain the optical properties of the monomer. Therefore, it is good to use powder pigments which leave an assignment to liquefy or micronize. Additionally, since the monomer has color, there is a problem that the pigment color and the color after molding becomes different.
When the grain size of the pigment is big, a comparatively uniform color of a pigment mixture can be obtained by mixing the silica particles for a long time and filtering out the silica particle afterwards. A mass production of uniform colored lenses can be achieved only when the filtering problem is solved.
2.3.1. Mixing Colors During Casting Process
Casting process line from the D company was used. After acquiring 1kg of the monomer raw material, 0.3g of a powder pigment was mixed in a small agitator for an hour, and then after filtering through a 0.3μm filter paper, it was injected to a ceramic lens mold for thermal polymerization of 24 hours in the 93℃ reacting unit. After making each 30 specimen each by using black and smog color and after investigating the color difference of each 10 specimens, the color was uniformly maintained in the , , range, but there was a slight difference in the required color of the pigment. This is because the original color exists in the material monomer which shall be studied further along with the color uniformity. Since there are lots of material monomer and pigment types, results for every cases are required.
2.3.2. Colored Lens for UV Protection
A plano lens was manufactured by mixing and thermal polymerizing the UV400 monomer (light yellow), a blue ink for high refractive index monomer and catalyst 27%. A middle tone brown was obtained and the UV protection rate was 97%.
2.4. Hard Coating Agent Pigment Mixing Method
This method is a slight transformation from the pigment agent wetting method. As the pigment agent wetting method mainly uses unprocessed lens, the surface hardness is weak, but this hard coating agent pigment mixing method has an advantage of enhancing the hardness. The desired color can be obtained but the processing time takes long. Using a similar method, there is a coloring method by dipping the hard coated lens into a pigment agent.
This may be referred to as a combination function of the lens and optic filter. Through the optical coating process on the lens surface while maintaining the colorless transparency of the lens, it is a method of designing a thin film to allow reflecting a certain color a lot. Although it is possible to design narrow spectral bandwidth, designing wide will be more natural to the eye considering the wide spectrum width of the sunlight. Essental 6.2 of the Macleo company was used for designing the thin film, and coating devices from P company was used for coating.
2.5.1. Characteristic Matrix of Thin Film
Characteristic matrix of thin film by multiple layers equals to sequentially multiplying each of the characteristic matrix.
M = M1M2M3…Mn (8)
Also, ignoring the absorption by the material, it can be said that the reflectance and transmittance mutually complementary.
T = 1-R (9)
If general lenses are focused on enhancing transmittance, the colored lens using thin film is focused on the reflectance as it need to depend on the reflection of a certain part of a light wave.
2.5.2. An Example of Designing Color Thin Film
A yellow filter was designed using ZnS and SiO2. 0° was used for the incident direction of light and 550nm for the reference wavelength. Figures 2.4.2 and 2.4.3 show the design value and the reflectance distribution by the design value. Controlling the number and thickness of layers, the position of the peak value can be adjusted, and so the color of the reflecting light can be controlled.
On the other hand, the color by the thin film can be changed according to the angle of the reflecting light. As the lens surface is spherical compared to a general filter which is flat, it is not fixed to a certain color, partially showing different colors.
3. Results and Discussion
Lens pigmentations were implemented through wetting, casting, vacuum deposition, etc. Also, digital camera was used instead of colorimeter for analyzing the colors of the specimens. Although the correction of the colorimetric values shall be done according to the equipment types, I would like to recommend using digital cameras which can be commonly used rather than costly colorimeter with limited usage. In the future, an industry or company that dominates color will win in the competition. The increase of colored lens supply which started from the young people emphasizes that a systematically research the relationship between the systematic research of colored lens and health relationship. As glass lenses are devices that are constantly worn, health and hygiene aspects shall be addressed along with the optical performance of lens. When wearing colored lens, the pupil dilates due to the reduction of light entering the eye. This leads to the increase of the entering UV light, so UV protection processed lens shall be worn.