Neutral Density Filters

In conditions where lighting is extremely bright, Neutral Density Filters can be essential. They generally appear gray in color and reduce the amount of light reaching the sensor without effecting color balance or contrast. They range from an optical density (OD) of 0.30 (ND030), which transmits about 50% of visible light to OD 1.20 (ND120), which transmits about 6.25 percent of visible light. In addition, polarizing filters can also function as Neutral Density Filters, with an OD of 0.50 (PR032), transmitting about 32 percent of available light. In machine vision applications, another common use for Neutral Density Filters is to decrease the depth of field by allowing wider lens apertures to be used. This helps separate subjects from their foreground and/or background as the subject will appears in focus, while the background will be out of focus. In sit-uations involving bright ambient light, or when cameras must be directed at high intensity beams, such as automotive headlamps, Neutral Density Filters reduce the amount of light captured by the CCD/CMOS imager.

IR & UV

Working in the NIR or UV spectrums offer significant advantages. Contrast in the NIR can be greatly improved depending on the characteristics of the item under inspection. 50 percent of the information captured in NIR images is significantly different when compared to those images captured with white light. This 50 percent rule can apply to any given type of subject. Unfortunately, it’s often impossible to tell how an image will appear in the NIR without experimentation. Fortunately, most CCD and CMOS cameras have excellent near-IR sensitivity, so tests can be performed quickly and easily by slipping a visible-blocking/IR pass filter over the camera’s lens. When a difference ap-pears, filters can provide optimum separation be-tween a subject and background.

In UV applications, 99 percent of inspections are not performed in the UV. These applications use UV or shorter wavelength lighting to excite a part or material under inspection. This material then emits light, usually somewhere in the visible portion of the spectrum. This fluorescence can often be weak and difficult to image. For such applications, a visible bandpass filters that’s appropriate for the emission wavelength is recommended. The bandpass filter should provide maximum transmission over the given wavelength range and block as much of the rest of the ambient light as possible – particularly the UV portion of the spectrum being emitted by the light source. With the use of CMOS cameras becoming widespread, a larger number of cameras now have significant UV sensitivity.

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