TECHNICAL INFORMATION

Published in "AEI December 2004"
Optical Low-Pass Filters Render Superb Quality Images in Cameras

 The popularity of digital cameras, which integrate image sensors such as CCD and CMOS, continues to surge such that the products have been touted as among the ‘Three New Sacred Treasures' in the digital home appliance field. As for digital camcorders, the market is steadily growing as orders keep coming in.
 Together with the rising demand for these products comes the requirement for exceptional quality of images rendered. In order to improve video quality, manufacturers are using optical low-pass filters (OLPFS) in cameras apart from a partial digital camera, and with the trend toward video digitization, the production of image sensors has been increasing.
 This article will discuss the application and technical trend of the optical low-pass filter used with sensor as a set.

Optical Low-Pass Filters

 In ordinary image sensors, the color filter arrays of RGB are arranged at equal intervals, and each RGB data is recognized in each pixel. In a pixel, only one color information can be recognized. Therefore, color information is reproduced from the information of vertical and horizontal pixels by calculation.
 However, a calculation error (the reflected distortion based on sampling) occurs when an input signal is insufficient, for example, when only a one-pixel video signal is received by the system. In this case, a pattern or coloring occurs in the portion where no video information exists originally. This causes a phenomenon called ‘pseudo-color (moiré)'.

Fig.1: Moire Phenomenon
Moire Phenomenon

 Thus, an optical low-pass filter is installed in front of an image sensor so as to cut the vicinity of an incidence light's sampling frequency (the pixel pitch frequency of a spatial frequency).
 There are several types of systems in an optical low-pass filter. In recent years, the system based on the double refraction characteristics (Fig.2) of artificial crystal has been almost used according to the conditions as follows: optical loss is few; characteristics are stable; the spatial frequency in the specific direction can be cut selectively; and price is low. Generally speaking, optical low-pass filter indicates a crystal system.

Principles of double refraction plate
Fig.2: Principles of double refraction plate

 At Daishinku Corp., using the autoclave (high-temperature and high-pressure vessel) made with special steel, artificial crystal is reared over several months under conditions of 400 deg.C and more than 1000 atmospheric pressures by a hydrothermal synthesis method. A double refraction plate with the maximum separation width is obtained from the high-quality artificial crystal reared for optical use by cutting down a crystal wafer at an angle of 45° with respect to the optical axis (Z-axis) of a crystal.

Digital Camera OLPFs

 For the image sensor used in digital cameras, the pixel pitch has also been miniaturized to satisfy the market requirements for high pixels and compactness. In a 4-million pixel sensor, which is the mainstream at present, the pixel pitch is approximately 2.5um. In the future, pixel pitch is expected to improve to about 2.0um.
 With the miniaturization of the pixel, design engineers have promoted a cut-off high frequency for the spatial frequency of an optical low-pass filter. The thickness of one crystal must be thinned to cut a higher frequency component. If simply calculated, the crystal thickness corresponding to a separation width of 2.5mm is 0.42mm. The crystal thickness corresponding to a separation width of 2.0mm is 0.34mm. In the image sensor for digital cameras, square pixels are generally arranged. For the design of a crystal optical low-pass filter, the square four-point separation configuration in which three crystals were attached together is mainly used to separate light into four pixels.
 The light permeated through a double refraction plate branches into ordinary and extraordinary rays. However, these rays are divided into the polarized component to which they are sent directly. Thus, two double refraction plates cannot theoretically configure square four-point separation. The square four-point separation can be obtained by combining the double refraction plate separated in the horizontal or vertical direction and two double refraction plates separated in the direction of 45 deg.C. In this case, crystal thickness T of the remaining two double refraction plates separated in the direction of 45 deg.C is T = Thickness of t / Å, ,2 when the crystal thickness of the double refraction plate separated in the horizontal or vertical direction is t. In this case, a relatively thin crystal plate is required.
 The use of an optical low-pass filter makes the formed image blurred and deteriorates the resolution. Consequently, a frequency that is even higher than the sampling frequency is cut off, and the optical low-pass filter is frequently designed by engineers keeping in mind the balance with resolution.
 Depending on specifications, multiple crystal wafers of less than 0.25mm may be attached and the final products may be cut out. Therefore, development of the technology that is produced when polishing technology, deposition technology, and adhesion technology are combined has become important.

OLPF of Camcorders

 CCD used for a camcorder has generally a longitudinal pixel. Therefore, the measures for moiré in the horizontal direction have priority. For the design of an optical low-pass filter, the parallelogram four-point separation configuration of two crystals or the horizontal two-point separation configuration of a single crystal is used mainly.
 In the future, however, a household camcorder will also be compatible with high-definition format. The number of crystals to be attached together will be increased for the improvement of the picture quality.

OLPF Optical Thin-film Technology
 In structure, a CCD/CMOS image sensor has sensitivity of ultraviolet rays to near infrared rays. Therefore, color reproduction different from a visible ray image that you view occurs when a color image is picked up directly.
 As a result, the unnecessary wavelength component of infrared rays that are received by a sensor must be cut. In general, the same function is added to an optical low-pass filter.
 In particular, the influence on an image by the ultraviolet rays such as chromatic aberration has been recently increasing because of the influence by the miniaturized pixel of an image sensor and the design limitation by the thinned lens. It has also become important to reduce ultraviolet rays.
 To cut infrared rays, there is a method for attaching infrared absorption glasses together or depositing infrared cut coating (Fig.3). There is also a way to deposit ultraviolet cut coating (Fig.4) when cutting ultraviolet rays.
 Infrared and ultraviolet cut coating (Fig.5) by which ultraviolet and infrared rays are simultaneously cut on one side has been employed recently.
 An image sensor or optical system is rapid in progress, and characteristics may often change in the process of progress. It is necessary to develop the cut-off characteristics of a thin film corresponding to the change in characteristics. Moreover, stable color reproduction is also requested. For dispersion specifications of spectral characteristics, ±8nm or more have also been requested as compared with conventional ±10nm.
 Since these specifications significantly influence the color reproducibility, the importance of optical thin-film technology has been increasing more and more.
 At Daishinku., a lot of characteristic variations are provided to satisfy different needs. Daishinku has also been preparing a development/production system in which importance is attached to the speed for satisfying new needs in a short time.

Hot Trend: SLR Digital Cameras

 The demand for single-lens reflex digital camera has been increasing as the requirement for pictures with superior quality increases. In the CCD and CMOS image sensor used, a large-sized sensor is used as compared with the sensor conforming to 35mm film or APS film size or the 4/3-inch sensor used for a general digital camera. As a result, a large-sized optical low-pass filter corresponding to the sensor used is also required.
 Since a high-resolution lens is used, the defect specifications of the optical low-pass filter installed in front of a CCD/CMOS sensor also require high-level quality. Depending on the products, the defect of a 10mm class may be also reflected on an actual image. Therefore, a skillful manufacturing technology that satisfies high-levels of external specifications in spite of a large-sized sensor is also required.
 In the future, Daishinku will attempt to keep on improving its processing technology and thin-film technology, and maintaining the system, which enables it to keep on supplying high-quality products at a low price. The company also aims to meet the needs for higher pixels and higher picture quality in digital cameras.

Spectral characteristics example of IR cut coating (AR coating on rear side)
Fig.3: Spectral characteristics example of IR cut coating (AR coating on rear side)
 
Spectral characteristics example of UV cut coating (AR coating on rear side)
Fig.4: Spectral characteristics example of UV cut coating (AR coating on rear side)
 
Spectral characteristics example of IR & UV cut coating (AR coating on rear side)
Fig.5: Spectral characteristics example of IR & UV cut coating (AR coating on rear side)

 

About This Article:
 The author, Taiku Kawashima, works at the Optical Device Production Division of Daishinku Corp.