Deferred income tax assets (Details) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Deferred Tax Assets [Line Items]
Losses carryforward (U.S.)	[1]	$ 6,984	$ 7,014
Losses carryforward (PRC)	[1]	2,767	2,000
Product warranties and other reserves		4,717	4,531
Property, plant and equipment		4,836	4,684
Share-based compensation		266	266
Bonus accrual		418	372
Other accruals		1,043	1,319
Others		1,544	1,496
Total deferred tax assets		22,575	21,682
Less: taxable temporary difference related to revenue recognition		(388)	(472)
Total deferred tax assets, net		22,187	21,210
Less: valuation allowance		(9,395)	(9,236)
Total deferred tax assets, net of valuation allowance	[2]	$ 12,792	$ 11,974

[1] The net operating losses carry forward for the U.S. entity for income tax purposes are available to reduce future years' taxable income. These losses will expire, if not utilized, in 20 years. Net operating losses carry forward for non-U.S. entities can be carried forward for 5 years to offset taxable income. However, as of June 30, 2015, the valuation allowance was $9.4 million, including $7.3 million allowance for the Company’s deferred tax assets in the United States and $2.1 million allowance for the Company’s non-U.S. deferred tax assets. Based on the Company’s current operations in the United States, management believes that the deferred tax assets in the United States are not likely to be realized in the future. For the non-U.S. deferred tax assets, pursuant to certain tax laws and regulations in China, the management believes such amount will not be used to offset future taxable income.

[2] Approximately $5.5 million and $4.9 million of net deferred income tax asset as of June 30, 2015 and December 31, 2014, respectively, are included in non-current deferred tax assets in the accompanying condensed unaudited consolidated balance sheets. The remaining $7.4 million and $7.1 million of net deferred income tax assets as of June 30, 2015 and December 31, 2014, respectively, are included in current deferred tax assets.