With special designed multi layer dielectric coating, the mirror reflects stated color      

while transmitting other wavelength at the same time.             

Dichroic mirror can be also used in SHG laser system as Harmanic Separators.               

Common Specifications:         

Substrate Material: BK7 or Fused silica             
Flatness: λ/4 commonly, λ/10 for laser application        
Surface quality: 20-10 (subatrate for laser application), 60-40 commonly         
Parallelism: < 1 arc minute      
Clear aperture: >85%      
Protective chamfer    
Front side dichroic coated        
Back side AR coated for transmitting wavelength          

Coating example for Laser SHG 1064-->532nm application:       
S1: HR R>99.5%@1064nm & HT T>90%@532nm, S2: AR R<0.25%@532nm, AOI=0deg.       
S1: HR R>99.5%@532nm & HT T>90%@1064nm, S2: AR R<0.25%@1064nm, AOI=0deg.  

Technical Information for reference：