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Intangible Assets
 12 Months Ended
Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangible Assets
7.  INTANGIBLE ASSETS
Patent costs, which are included in Other assets in the accompanying Consolidated Balance Sheets, as of December 31 were as follows (in thousands):
 
2019
 
  
2018
 
Patent costs
  $
  1,992
     $
  1,979
  
Accumulated amortization
   
(1,483
 )    
(1,380
 )
                 
   $
509
     $
599
  
                 
Definite lived intangible assets, such as patent rights, are amortized and tested for impairment if a triggering event occurs.
Amortization expense was approximately $108,000, $119,000 and $130,000 in 2019, 2018 and 2017, respectively. The estimated future amortization expense from patent assets held as of December 31, 2019, is projected to be $104,000, $95,000, $63,000, $52,000 and $44,000, in fiscal years 2020, 2021, 2022, 2023, and 2024, respectively.