Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Accounts receivable, due to allowance for doubtful accounts	$ 373	$ 577
Inventories	776	718
Operating loss carry-forwards	1,343	3,064
Stock-based compensation expense	807	762
Property and equipment, due to difference in depreciation	47	53
Research and development, alternative minimum tax credit carry-forwards	5,243	4,716
Foreign tax credit carry-forwards	2,345	2,360
State tax credit carry-forwards	3,146	2,977
Capitalized research and development	3,263	3,130
Warranty reserve	523	454
Accrued expenses	845	556
Deferred Tax Asset, Right-of-Use Assets	1,378
Gross deferred tax assets	20,089	19,367
Less valuation allowance	(18,452)	(18,144)
Total deferred tax assets	1,637	1,223
Deferred tax liabilities:
Purchased intangible assets	(844)	(951)
Property and equipment, due to differences in depreciation	(132)	(994)
Deferred Lease Liability, Lease Liability	(1,378)
Other	0	(18)
Total deferred tax liabilities	(2,354)	(1,963)
Net deferred tax liability	(717)	(740)
Non-current deferred income tax asset	45	226
Non-current deferred income tax liability	$ (762)	$ (966)