Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Deferred tax assets:
Accounts receivable, due to allowance for doubtful accounts	$ 582		$ 540
Inventories	718		581
Operating loss carry-forwards	3,064		4,725
Stock-based compensation expense	762		696
Property and equipment, due to difference in depreciation	173		190
Research and development, alternative minimum tax credit carry-forwards	4,716		4,338
Foreign tax credit carry-forwards	2,360		2,958
State tax credit carry-forwards	2,977		2,958
Capitalized research and development	3,130		0
Warranty reserve	454		495
Accrued expenses	556		334
Gross deferred tax assets	19,492		17,815
Less valuation allowance	(18,144)		(16,014)
Total deferred tax assets	1,348		1,801
Deferred tax liabilities:
Purchased intangible assets	(1,810)		(2,705)
Property and equipment, due to differences in depreciation	(994)		(1,681)
Other	(18)		(29)
Total deferred tax liabilities	(2,822)		(4,415)
Net deferred tax liability	(1,474)		(2,614)
Net deferred tax asset- non-current	226	$ 222	20
Net deferred tax liability- non-current	$ (1,700)		$ (2,634)