Revenue (Tables)

6 Months Ended

Jun. 30, 2019

Revenue.

Schedule of revenue by discipline

	Three months ended				Six months ended
	June 30,				June 30,
	2019		2018		2019		2018
	(in thousands)
Systems	$	37,224	$	73,919	$	94,316	$	159,395
Aftermarket		37,081		45,414		71,466		82,123
	$	74,305	$	119,333	$	165,782	$	241,518

Schedule of revenue by geographic markets

	Three months ended				Six months ended
	June 30,				June 30,
	2019		2018		2019		2018
	(in thousands)
North America	$	9,380	$	16,485	$	20,988	$	27,008
Asia Pacific		49,717		88,471		112,643		186,160
Europe		15,208		14,377		32,151		28,350
	$	74,305	$	119,333	$	165,782	$	241,518

Schedule of contract assets and contract liabilities

	June 30,		December 31,
	2019		2018
	(in thousands)
Contract assets	$	—	$	—
Contract liabilities	$	14,080	$	22,584

Contract liabilities are reflected as deferred revenue on the consolidated balance sheet and relate to payments invoiced or received in advance of completion of performance obligations under a contract. Contract liabilities are recognized as revenue upon the fulfillment of performance obligations.

	Three months ended				Six months ended
	June 30,				June 30,
	2019		2018		2019		2018
	(in thousands)
Balance, beginning of the period	$	20,481	$	18,522	$	22,584	$	18,145
Deferral of revenue		3,943		5,440		6,225		7,647
Recognition of deferred revenue		(10,344)		(8,787)		(14,729)		(10,617)
Balance, ending of the period	$	14,080	$	15,175	$	14,080	$	15,175