CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
CONSOLIDATED BALANCE SHEETS
Accounts receivable, net of allowance for doubtful accounts	$ 41	$ 36
Patents and licenses, net of accumulated amortization	$ 12,419	$ 12,301
Stockholders' equity
Common stock, shares par value	$ 0.10	$ 0.1
Common stock, shares authorized	10,000	10,000
Common stock, shares issued	3,420	3,420
Treasury stock, shares	1,594	1,565