Neutral density (ND) filters are used to attenuate incident radiation without altering its spectral distribution. The neutral density value of an ND filter is related to the transmittance by:            

OD=Log₁₀(1/T)    or T=10^-OD         
Optical Density: OD              

Transmittance: T                        

Absorptive Neutral Density (ND) Filters

These filters possess level spectral transmittance characteristics in the visible region and attenuate light by absorption with minimal reflection. Typically, neutrality and density of absorptive filters are a function of the material and the thickness. Spectral variations occur as optical density increases.

Common Specifications           
Material: Optical Neutral Density Glass            
Flatness:  λ /2   per 25 mm                    
Surface Quality: 60/40         
Parallelism: 10 arc sec                     
Dimension Tolerance: +/-0.2 mm         
Optical Density Tolerance : ± 5%      

Technical curve

                                                                                         Chinese glass

                                                                                                        Schott glass

We also provide AR coating on the filter. The high-performance, multilayer broadband AR coating deposited on each filter is designed to minimize surface reflections within the 350 - 700 nm range( or other requested wavelength range), thereby reducing the amount of stray light present. These coatings provide an average reflectivity that is less than 0.5% over the specified wavelength range when light is normally incident on the filter surface.   

  

Reflective (Metallic Coated )Neutral Density Filter

Metallic coated ND filters are glass or quartz substrates with a precision metallic coating (Inconel) that provides uniform attenuation across a wide spectral range. Inconel is resistant to aging under normal conditions; however, it will oxidize at elevated temperatures. To prevent oxidation, Tempotec recommends using these ND filters at temperatures below 100 °C.          

Common Specifications     

Attenuation for the 200 - 1200 nm Range

Optical Densities from 0.1 to 4.0
Material: BK7 glass and Fused Silica
Flatness: 2λ per 25 mm
Surface Quality: 60/40
Parallelism: 10 arc sec
Dimension Tolerance: +/-0.2 mm
Coating: Vacuum-deposited metallic alloy
Optical density Tolerance: +/- 5%

 

Technical curve