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Summary of Significant Accounting Policies - Accounts Receivable, net (Details) - USD ($)
Mar. 31, 2019
Dec. 31, 2018
Accounts receivable    
Allowance for product returns $ 96,000 $ 144,000
Allowance for price concessions 26,000 569,000
Trade accounts receivable 5,077,000 7,297,000
Unbilled accounts receivable 1,027,000 938,000
Allowance for accounts receivable (122,000) (713,000)
Accounts receivable, net $ 5,982,000 $ 7,522,000