Neutral Density Filter

Best viewed in Internet Explorer 5 and Netscape 4.7

PRODUCTS
	Accessories
	Fiber Optics & Spectrometer
	Educational Kits
	Light Source
	Optics
	Opto-Electronics
	Opto-Mechanics
	Software

PLACES TO GO
	About SmartSense
	Contact SmartSense
	Site Map
	Knowledge Center
	Useful Links
Email: [email protected]

HOME	\|	BACK
NEUTRAL DENSITY FILTER (ND FILTER)

Neutral density filters are usually used to reduce the intensity of light and is useful in light control applications for measuring instruments and exposure control in imaging. Applications range from:

Reducing light intensity without affecting spectral response

Obtaining reading that are truncated

Protect imaging equipment from over exposure

The optical density, OD, of the ND filter is defined using the following equation:

where		: intensity of incident ray
		: intensity of transmitted ray

Neutral density filter achieves attenuation in 2 ways:

Reflection

Absorption of unwanted light

Absorptive ND Filters
These filters rely on absorption of unwanted light as the attenuation mechanism. When used in multiple stack, you could achieve any desirable light intensity level and at the same time, without affecting the spectral response of light from the source. The total optical density is calculated the summation of each individual filter's density, which is a plus points for absorptive filters over reflective filters. However, the main disadvantage is that it does not give as good response curve as its counterpart - reflective ND filters, making calibration necessary for precise experiments. See experimental techniques for more details on how to achieve it.

Reflective ND Filters
As the name suggested, reflective ND filters reflect the unwanted light and only allows a certain percentage of light passing through it. These filters made use of a metallic coating (lnconel) that were applied onto a flat polished substrate that is usually fused quartz. The properties of Inconel is such the spectral response for transmission could be maintained over a much wider range as compared to absorptive type. The main advantage of this filter is that its transmission (%) has a very flat response over a wide spectral range, making it suitable for general purposes usage.

Technical Advice

As the coated lnconel will oxidize at elevated temperature or over time, affecting the optical density value, hence it is recommended to use it at temperatures below 100°C.

As the value of OD is measured and true at a particular calibrated value only, there will be a slight deviation from the claimed value if used at other wavelength. Hence if you desire to obtain the actual OD for your working wavelength, calibration is required. See experimental techniques for more details on how to achieve.

Though the optical densities could be combined additively in stacks, this is not advisable for reflective ND filter due to the multiple reflection effects.

Typically, OD value is also a function of glass type and thickness of the filter.

Hosted by www.Geocities.ws